Re: [Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
On 09/05/2018 06:27 AM, Max Reitz wrote:
> On 2018-09-04 22:32, John Snow wrote:
>>
>>
>> On 09/04/2018 02:46 PM, Jeff Cody wrote:
>>> On Tue, Sep 04, 2018 at 01:09:19PM -0400, John Snow wrote:
Use the component callbacks; prepare, abort, and clean.
NB: prepare is only called when the job has not yet failed;
and abort can be called after prepare.
complete -> prepare -> abort -> clean
complete -> abort -> clean
Signed-off-by: John Snow
Reviewed-by: Max Reitz
---
block/commit.c | 90
--
1 file changed, 49 insertions(+), 41 deletions(-)
diff --git a/block/commit.c b/block/commit.c
index b6e8969877..eb3941e545 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
BlockDriverState *commit_top_bs;
BlockBackend *top;
BlockBackend *base;
+BlockDriverState *base_bs;
BlockdevOnError on_error;
int base_flags;
char *backing_file_str;
@@ -68,61 +69,65 @@ static int coroutine_fn commit_populate(BlockBackend
*bs, BlockBackend *base,
return 0;
}
-static void commit_exit(Job *job)
+static int commit_prepare(Job *job)
{
CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
-BlockJob *bjob = >common;
-BlockDriverState *top = blk_bs(s->top);
-BlockDriverState *base = blk_bs(s->base);
-BlockDriverState *commit_top_bs = s->commit_top_bs;
-bool remove_commit_top_bs = false;
-
-/* Make sure commit_top_bs and top stay around until
bdrv_replace_node() */
-bdrv_ref(top);
-bdrv_ref(commit_top_bs);
/* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE
before
* the normal backing chain can be restored. */
blk_unref(s->base);
+s->base = NULL;
-if (!job_is_cancelled(job) && job->ret == 0) {
-/* success */
-job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
- s->backing_file_str);
-} else {
-/* XXX Can (or should) we somehow keep 'consistent read' blocked
even
- * after the failed/cancelled commit job is gone? If we already
wrote
- * something to base, the intermediate images aren't valid any
more. */
-remove_commit_top_bs = true;
+return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
+ s->backing_file_str);
+}
>>>
>>> If we can go from prepare->abort->clean, then that means to me that every
>>> failure case of .prepare() can be resolved without permanent changes / data
>>> loss. Is this necessarily the case?
>>>
>>
>> That'd be a requisite to make the job a transaction, but commit, mirror
>> and stream are not currently transactionable.
>
> Is that already documented anywhere?
>
Hm, no, not really.
I'm most inclined to document it near the action table because it would
be hard to miss if you went to add it.
I'll add this in an extra patch at the end in case you want to debate
the wording and/or location.
> (Otherwise I'd be afraid of us forgetting in like a year, asking "Why
> isn't this a transaction already?", just making it one, and then
> remembering half a year later.)
>
No, it's a good point.
>> The way commit already works, for example, can leave the base and
>> intermediate images as unusable as standalone images. This refactoring
>> will not change that alone.
>>
>> So it's not necessarily a problem, but it's something that would need to
>> be fixed if we ever wanted transaction support.
>>
>> However, in talking on IRC we did realize that this patch does change
>> behavior...
>>
>> Before:
>>
>> If bdrv_drop_intermediate fails, we store the retcode but continue
>> cleaning up as if it didn't fail. i.e., we don't remove the commit job's
>> installed top_bs node.
>>
>> After:
>>
>> if bdrv_drop_intermediate fails, we return the failure retcode and
>> .abort gets called as a result, i.e. we will remove the commit job's
>> installed top_bs node in favor of the original top_bs node.
>>
>> I think this behavior is an improvement,
>
> I agree.
>
Based on this I will leave the stickier fix to a future patch... I will
add a FIXME note detailing the shortfall in this patch, but I am
asserting that the current behavior is /not worse/ than the old
behavior, while there is still a bug that we might need to fix in the
future.
>> however it raises a question
>> about the nature of failures in bdrv_drop_intermediate.
>>
>> If this function fails without making any changes, the new commit
>> behavior is good. If it succeeds, we're also good. The problem is with
>> intermediate or partial successes.
>>
>> If top has
Re: [Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
On 2018-09-05 13:53, Kevin Wolf wrote:
> Am 05.09.2018 um 13:37 hat Max Reitz geschrieben:
>> On 2018-09-05 12:49, Kevin Wolf wrote:
>>> Am 05.09.2018 um 12:27 hat Max Reitz geschrieben:
On 2018-09-04 22:32, John Snow wrote:
>
>
> On 09/04/2018 02:46 PM, Jeff Cody wrote:
>> On Tue, Sep 04, 2018 at 01:09:19PM -0400, John Snow wrote:
>>> Use the component callbacks; prepare, abort, and clean.
>>>
>>> NB: prepare is only called when the job has not yet failed;
>>> and abort can be called after prepare.
>>>
>>> complete -> prepare -> abort -> clean
>>> complete -> abort -> clean
>>>
>>> Signed-off-by: John Snow
>>> Reviewed-by: Max Reitz
>>> ---
>>> block/commit.c | 90
>>> --
>>> 1 file changed, 49 insertions(+), 41 deletions(-)
>>>
>>> diff --git a/block/commit.c b/block/commit.c
>>> index b6e8969877..eb3941e545 100644
>>> --- a/block/commit.c
>>> +++ b/block/commit.c
>>> @@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
>>> BlockDriverState *commit_top_bs;
>>> BlockBackend *top;
>>> BlockBackend *base;
>>> +BlockDriverState *base_bs;
>>> BlockdevOnError on_error;
>>> int base_flags;
>>> char *backing_file_str;
>>> @@ -68,61 +69,65 @@ static int coroutine_fn
>>> commit_populate(BlockBackend *bs, BlockBackend *base,
>>> return 0;
>>> }
>>>
>>> -static void commit_exit(Job *job)
>>> +static int commit_prepare(Job *job)
>>> {
>>> CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
>>> -BlockJob *bjob = >common;
>>> -BlockDriverState *top = blk_bs(s->top);
>>> -BlockDriverState *base = blk_bs(s->base);
>>> -BlockDriverState *commit_top_bs = s->commit_top_bs;
>>> -bool remove_commit_top_bs = false;
>>> -
>>> -/* Make sure commit_top_bs and top stay around until
>>> bdrv_replace_node() */
>>> -bdrv_ref(top);
>>> -bdrv_ref(commit_top_bs);
>>>
>>> /* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE
>>> before
>>> * the normal backing chain can be restored. */
>>> blk_unref(s->base);
>>> +s->base = NULL;
>>>
>>> -if (!job_is_cancelled(job) && job->ret == 0) {
>>> -/* success */
>>> -job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
>>> - s->backing_file_str);
>>> -} else {
>>> -/* XXX Can (or should) we somehow keep 'consistent read'
>>> blocked even
>>> - * after the failed/cancelled commit job is gone? If we
>>> already wrote
>>> - * something to base, the intermediate images aren't valid any
>>> more. */
>>> -remove_commit_top_bs = true;
>>> +return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
>>> + s->backing_file_str);
>>> +}
>>
>> If we can go from prepare->abort->clean, then that means to me that every
>> failure case of .prepare() can be resolved without permanent changes /
>> data
>> loss. Is this necessarily the case?
>>
>
> That'd be a requisite to make the job a transaction, but commit, mirror
> and stream are not currently transactionable.
Is that already documented anywhere?
(Otherwise I'd be afraid of us forgetting in like a year, asking "Why
isn't this a transaction already?", just making it one, and then
remembering half a year later.)
> The way commit already works, for example, can leave the base and
> intermediate images as unusable as standalone images. This refactoring
> will not change that alone.
>
> So it's not necessarily a problem, but it's something that would need to
> be fixed if we ever wanted transaction support.
>
> However, in talking on IRC we did realize that this patch does change
> behavior...
>
> Before:
>
> If bdrv_drop_intermediate fails, we store the retcode but continue
> cleaning up as if it didn't fail. i.e., we don't remove the commit job's
> installed top_bs node.
>
> After:
>
> if bdrv_drop_intermediate fails, we return the failure retcode and
> .abort gets called as a result, i.e. we will remove the commit job's
> installed top_bs node in favor of the original top_bs node.
>
> I think this behavior is an improvement,
I agree.
> however it raises a question
> about the nature of failures in bdrv_drop_intermediate.
>
> If this function fails without making any changes, the new commit
> behavior is good. If it succeeds, we're also good. The problem is with
> intermediate or partial successes.
>
> If top has multiple parents (I think
Re: [Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
Am 05.09.2018 um 13:37 hat Max Reitz geschrieben:
> On 2018-09-05 12:49, Kevin Wolf wrote:
> > Am 05.09.2018 um 12:27 hat Max Reitz geschrieben:
> >> On 2018-09-04 22:32, John Snow wrote:
> >>>
> >>>
> >>> On 09/04/2018 02:46 PM, Jeff Cody wrote:
> On Tue, Sep 04, 2018 at 01:09:19PM -0400, John Snow wrote:
> > Use the component callbacks; prepare, abort, and clean.
> >
> > NB: prepare is only called when the job has not yet failed;
> > and abort can be called after prepare.
> >
> > complete -> prepare -> abort -> clean
> > complete -> abort -> clean
> >
> > Signed-off-by: John Snow
> > Reviewed-by: Max Reitz
> > ---
> > block/commit.c | 90
> > --
> > 1 file changed, 49 insertions(+), 41 deletions(-)
> >
> > diff --git a/block/commit.c b/block/commit.c
> > index b6e8969877..eb3941e545 100644
> > --- a/block/commit.c
> > +++ b/block/commit.c
> > @@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
> > BlockDriverState *commit_top_bs;
> > BlockBackend *top;
> > BlockBackend *base;
> > +BlockDriverState *base_bs;
> > BlockdevOnError on_error;
> > int base_flags;
> > char *backing_file_str;
> > @@ -68,61 +69,65 @@ static int coroutine_fn
> > commit_populate(BlockBackend *bs, BlockBackend *base,
> > return 0;
> > }
> >
> > -static void commit_exit(Job *job)
> > +static int commit_prepare(Job *job)
> > {
> > CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
> > -BlockJob *bjob = >common;
> > -BlockDriverState *top = blk_bs(s->top);
> > -BlockDriverState *base = blk_bs(s->base);
> > -BlockDriverState *commit_top_bs = s->commit_top_bs;
> > -bool remove_commit_top_bs = false;
> > -
> > -/* Make sure commit_top_bs and top stay around until
> > bdrv_replace_node() */
> > -bdrv_ref(top);
> > -bdrv_ref(commit_top_bs);
> >
> > /* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE
> > before
> > * the normal backing chain can be restored. */
> > blk_unref(s->base);
> > +s->base = NULL;
> >
> > -if (!job_is_cancelled(job) && job->ret == 0) {
> > -/* success */
> > -job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
> > - s->backing_file_str);
> > -} else {
> > -/* XXX Can (or should) we somehow keep 'consistent read'
> > blocked even
> > - * after the failed/cancelled commit job is gone? If we
> > already wrote
> > - * something to base, the intermediate images aren't valid any
> > more. */
> > -remove_commit_top_bs = true;
> > +return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
> > + s->backing_file_str);
> > +}
>
> If we can go from prepare->abort->clean, then that means to me that every
> failure case of .prepare() can be resolved without permanent changes /
> data
> loss. Is this necessarily the case?
>
> >>>
> >>> That'd be a requisite to make the job a transaction, but commit, mirror
> >>> and stream are not currently transactionable.
> >>
> >> Is that already documented anywhere?
> >>
> >> (Otherwise I'd be afraid of us forgetting in like a year, asking "Why
> >> isn't this a transaction already?", just making it one, and then
> >> remembering half a year later.)
> >>
> >>> The way commit already works, for example, can leave the base and
> >>> intermediate images as unusable as standalone images. This refactoring
> >>> will not change that alone.
> >>>
> >>> So it's not necessarily a problem, but it's something that would need to
> >>> be fixed if we ever wanted transaction support.
> >>>
> >>> However, in talking on IRC we did realize that this patch does change
> >>> behavior...
> >>>
> >>> Before:
> >>>
> >>> If bdrv_drop_intermediate fails, we store the retcode but continue
> >>> cleaning up as if it didn't fail. i.e., we don't remove the commit job's
> >>> installed top_bs node.
> >>>
> >>> After:
> >>>
> >>> if bdrv_drop_intermediate fails, we return the failure retcode and
> >>> .abort gets called as a result, i.e. we will remove the commit job's
> >>> installed top_bs node in favor of the original top_bs node.
> >>>
> >>> I think this behavior is an improvement,
> >>
> >> I agree.
> >>
> >>> however it raises a question
> >>> about the nature of failures in bdrv_drop_intermediate.
> >>>
> >>> If this function fails without making any changes, the new commit
> >>> behavior is good. If it succeeds, we're also good. The problem is with
> >>> intermediate or partial successes.
> >>>
> >>> If top has multiple parents (I think under normal circumstances it
> >>> won't,
Re: [Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
On 2018-09-05 12:49, Kevin Wolf wrote:
> Am 05.09.2018 um 12:27 hat Max Reitz geschrieben:
>> On 2018-09-04 22:32, John Snow wrote:
>>>
>>>
>>> On 09/04/2018 02:46 PM, Jeff Cody wrote:
On Tue, Sep 04, 2018 at 01:09:19PM -0400, John Snow wrote:
> Use the component callbacks; prepare, abort, and clean.
>
> NB: prepare is only called when the job has not yet failed;
> and abort can be called after prepare.
>
> complete -> prepare -> abort -> clean
> complete -> abort -> clean
>
> Signed-off-by: John Snow
> Reviewed-by: Max Reitz
> ---
> block/commit.c | 90
> --
> 1 file changed, 49 insertions(+), 41 deletions(-)
>
> diff --git a/block/commit.c b/block/commit.c
> index b6e8969877..eb3941e545 100644
> --- a/block/commit.c
> +++ b/block/commit.c
> @@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
> BlockDriverState *commit_top_bs;
> BlockBackend *top;
> BlockBackend *base;
> +BlockDriverState *base_bs;
> BlockdevOnError on_error;
> int base_flags;
> char *backing_file_str;
> @@ -68,61 +69,65 @@ static int coroutine_fn commit_populate(BlockBackend
> *bs, BlockBackend *base,
> return 0;
> }
>
> -static void commit_exit(Job *job)
> +static int commit_prepare(Job *job)
> {
> CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
> -BlockJob *bjob = >common;
> -BlockDriverState *top = blk_bs(s->top);
> -BlockDriverState *base = blk_bs(s->base);
> -BlockDriverState *commit_top_bs = s->commit_top_bs;
> -bool remove_commit_top_bs = false;
> -
> -/* Make sure commit_top_bs and top stay around until
> bdrv_replace_node() */
> -bdrv_ref(top);
> -bdrv_ref(commit_top_bs);
>
> /* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE
> before
> * the normal backing chain can be restored. */
> blk_unref(s->base);
> +s->base = NULL;
>
> -if (!job_is_cancelled(job) && job->ret == 0) {
> -/* success */
> -job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
> - s->backing_file_str);
> -} else {
> -/* XXX Can (or should) we somehow keep 'consistent read' blocked
> even
> - * after the failed/cancelled commit job is gone? If we already
> wrote
> - * something to base, the intermediate images aren't valid any
> more. */
> -remove_commit_top_bs = true;
> +return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
> + s->backing_file_str);
> +}
If we can go from prepare->abort->clean, then that means to me that every
failure case of .prepare() can be resolved without permanent changes / data
loss. Is this necessarily the case?
>>>
>>> That'd be a requisite to make the job a transaction, but commit, mirror
>>> and stream are not currently transactionable.
>>
>> Is that already documented anywhere?
>>
>> (Otherwise I'd be afraid of us forgetting in like a year, asking "Why
>> isn't this a transaction already?", just making it one, and then
>> remembering half a year later.)
>>
>>> The way commit already works, for example, can leave the base and
>>> intermediate images as unusable as standalone images. This refactoring
>>> will not change that alone.
>>>
>>> So it's not necessarily a problem, but it's something that would need to
>>> be fixed if we ever wanted transaction support.
>>>
>>> However, in talking on IRC we did realize that this patch does change
>>> behavior...
>>>
>>> Before:
>>>
>>> If bdrv_drop_intermediate fails, we store the retcode but continue
>>> cleaning up as if it didn't fail. i.e., we don't remove the commit job's
>>> installed top_bs node.
>>>
>>> After:
>>>
>>> if bdrv_drop_intermediate fails, we return the failure retcode and
>>> .abort gets called as a result, i.e. we will remove the commit job's
>>> installed top_bs node in favor of the original top_bs node.
>>>
>>> I think this behavior is an improvement,
>>
>> I agree.
>>
>>> however it raises a question
>>> about the nature of failures in bdrv_drop_intermediate.
>>>
>>> If this function fails without making any changes, the new commit
>>> behavior is good. If it succeeds, we're also good. The problem is with
>>> intermediate or partial successes.
>>>
>>> If top has multiple parents (I think under normal circumstances it
>>> won't, but I'm not absolutely sure) and it fails to update their backing
>>> file references, it might partially succeed.
>>>
>>> I think commit's usage here is correct, but I think we might need to
>>> update bdrv_drop_intermediate to make it roll back changes if it
>>> experiences a partial failure to give
Re: [Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
Am 05.09.2018 um 12:27 hat Max Reitz geschrieben:
> On 2018-09-04 22:32, John Snow wrote:
> >
> >
> > On 09/04/2018 02:46 PM, Jeff Cody wrote:
> >> On Tue, Sep 04, 2018 at 01:09:19PM -0400, John Snow wrote:
> >>> Use the component callbacks; prepare, abort, and clean.
> >>>
> >>> NB: prepare is only called when the job has not yet failed;
> >>> and abort can be called after prepare.
> >>>
> >>> complete -> prepare -> abort -> clean
> >>> complete -> abort -> clean
> >>>
> >>> Signed-off-by: John Snow
> >>> Reviewed-by: Max Reitz
> >>> ---
> >>> block/commit.c | 90
> >>> --
> >>> 1 file changed, 49 insertions(+), 41 deletions(-)
> >>>
> >>> diff --git a/block/commit.c b/block/commit.c
> >>> index b6e8969877..eb3941e545 100644
> >>> --- a/block/commit.c
> >>> +++ b/block/commit.c
> >>> @@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
> >>> BlockDriverState *commit_top_bs;
> >>> BlockBackend *top;
> >>> BlockBackend *base;
> >>> +BlockDriverState *base_bs;
> >>> BlockdevOnError on_error;
> >>> int base_flags;
> >>> char *backing_file_str;
> >>> @@ -68,61 +69,65 @@ static int coroutine_fn commit_populate(BlockBackend
> >>> *bs, BlockBackend *base,
> >>> return 0;
> >>> }
> >>>
> >>> -static void commit_exit(Job *job)
> >>> +static int commit_prepare(Job *job)
> >>> {
> >>> CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
> >>> -BlockJob *bjob = >common;
> >>> -BlockDriverState *top = blk_bs(s->top);
> >>> -BlockDriverState *base = blk_bs(s->base);
> >>> -BlockDriverState *commit_top_bs = s->commit_top_bs;
> >>> -bool remove_commit_top_bs = false;
> >>> -
> >>> -/* Make sure commit_top_bs and top stay around until
> >>> bdrv_replace_node() */
> >>> -bdrv_ref(top);
> >>> -bdrv_ref(commit_top_bs);
> >>>
> >>> /* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE
> >>> before
> >>> * the normal backing chain can be restored. */
> >>> blk_unref(s->base);
> >>> +s->base = NULL;
> >>>
> >>> -if (!job_is_cancelled(job) && job->ret == 0) {
> >>> -/* success */
> >>> -job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
> >>> - s->backing_file_str);
> >>> -} else {
> >>> -/* XXX Can (or should) we somehow keep 'consistent read' blocked
> >>> even
> >>> - * after the failed/cancelled commit job is gone? If we already
> >>> wrote
> >>> - * something to base, the intermediate images aren't valid any
> >>> more. */
> >>> -remove_commit_top_bs = true;
> >>> +return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
> >>> + s->backing_file_str);
> >>> +}
> >>
> >> If we can go from prepare->abort->clean, then that means to me that every
> >> failure case of .prepare() can be resolved without permanent changes / data
> >> loss. Is this necessarily the case?
> >>
> >
> > That'd be a requisite to make the job a transaction, but commit, mirror
> > and stream are not currently transactionable.
>
> Is that already documented anywhere?
>
> (Otherwise I'd be afraid of us forgetting in like a year, asking "Why
> isn't this a transaction already?", just making it one, and then
> remembering half a year later.)
>
> > The way commit already works, for example, can leave the base and
> > intermediate images as unusable as standalone images. This refactoring
> > will not change that alone.
> >
> > So it's not necessarily a problem, but it's something that would need to
> > be fixed if we ever wanted transaction support.
> >
> > However, in talking on IRC we did realize that this patch does change
> > behavior...
> >
> > Before:
> >
> > If bdrv_drop_intermediate fails, we store the retcode but continue
> > cleaning up as if it didn't fail. i.e., we don't remove the commit job's
> > installed top_bs node.
> >
> > After:
> >
> > if bdrv_drop_intermediate fails, we return the failure retcode and
> > .abort gets called as a result, i.e. we will remove the commit job's
> > installed top_bs node in favor of the original top_bs node.
> >
> > I think this behavior is an improvement,
>
> I agree.
>
> > however it raises a question
> > about the nature of failures in bdrv_drop_intermediate.
> >
> > If this function fails without making any changes, the new commit
> > behavior is good. If it succeeds, we're also good. The problem is with
> > intermediate or partial successes.
> >
> > If top has multiple parents (I think under normal circumstances it
> > won't, but I'm not absolutely sure) and it fails to update their backing
> > file references, it might partially succeed.
> >
> > I think commit's usage here is correct, but I think we might need to
> > update bdrv_drop_intermediate to make it roll back changes if it
> > experiences a partial failure to give all-or-nothing semantics.
>
> Sure,
Re: [Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
On 2018-09-04 22:32, John Snow wrote:
>
>
> On 09/04/2018 02:46 PM, Jeff Cody wrote:
>> On Tue, Sep 04, 2018 at 01:09:19PM -0400, John Snow wrote:
>>> Use the component callbacks; prepare, abort, and clean.
>>>
>>> NB: prepare is only called when the job has not yet failed;
>>> and abort can be called after prepare.
>>>
>>> complete -> prepare -> abort -> clean
>>> complete -> abort -> clean
>>>
>>> Signed-off-by: John Snow
>>> Reviewed-by: Max Reitz
>>> ---
>>> block/commit.c | 90
>>> --
>>> 1 file changed, 49 insertions(+), 41 deletions(-)
>>>
>>> diff --git a/block/commit.c b/block/commit.c
>>> index b6e8969877..eb3941e545 100644
>>> --- a/block/commit.c
>>> +++ b/block/commit.c
>>> @@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
>>> BlockDriverState *commit_top_bs;
>>> BlockBackend *top;
>>> BlockBackend *base;
>>> +BlockDriverState *base_bs;
>>> BlockdevOnError on_error;
>>> int base_flags;
>>> char *backing_file_str;
>>> @@ -68,61 +69,65 @@ static int coroutine_fn commit_populate(BlockBackend
>>> *bs, BlockBackend *base,
>>> return 0;
>>> }
>>>
>>> -static void commit_exit(Job *job)
>>> +static int commit_prepare(Job *job)
>>> {
>>> CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
>>> -BlockJob *bjob = >common;
>>> -BlockDriverState *top = blk_bs(s->top);
>>> -BlockDriverState *base = blk_bs(s->base);
>>> -BlockDriverState *commit_top_bs = s->commit_top_bs;
>>> -bool remove_commit_top_bs = false;
>>> -
>>> -/* Make sure commit_top_bs and top stay around until
>>> bdrv_replace_node() */
>>> -bdrv_ref(top);
>>> -bdrv_ref(commit_top_bs);
>>>
>>> /* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE before
>>> * the normal backing chain can be restored. */
>>> blk_unref(s->base);
>>> +s->base = NULL;
>>>
>>> -if (!job_is_cancelled(job) && job->ret == 0) {
>>> -/* success */
>>> -job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
>>> - s->backing_file_str);
>>> -} else {
>>> -/* XXX Can (or should) we somehow keep 'consistent read' blocked
>>> even
>>> - * after the failed/cancelled commit job is gone? If we already
>>> wrote
>>> - * something to base, the intermediate images aren't valid any
>>> more. */
>>> -remove_commit_top_bs = true;
>>> +return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
>>> + s->backing_file_str);
>>> +}
>>
>> If we can go from prepare->abort->clean, then that means to me that every
>> failure case of .prepare() can be resolved without permanent changes / data
>> loss. Is this necessarily the case?
>>
>
> That'd be a requisite to make the job a transaction, but commit, mirror
> and stream are not currently transactionable.
Is that already documented anywhere?
(Otherwise I'd be afraid of us forgetting in like a year, asking "Why
isn't this a transaction already?", just making it one, and then
remembering half a year later.)
> The way commit already works, for example, can leave the base and
> intermediate images as unusable as standalone images. This refactoring
> will not change that alone.
>
> So it's not necessarily a problem, but it's something that would need to
> be fixed if we ever wanted transaction support.
>
> However, in talking on IRC we did realize that this patch does change
> behavior...
>
> Before:
>
> If bdrv_drop_intermediate fails, we store the retcode but continue
> cleaning up as if it didn't fail. i.e., we don't remove the commit job's
> installed top_bs node.
>
> After:
>
> if bdrv_drop_intermediate fails, we return the failure retcode and
> .abort gets called as a result, i.e. we will remove the commit job's
> installed top_bs node in favor of the original top_bs node.
>
> I think this behavior is an improvement,
I agree.
> however it raises a question
> about the nature of failures in bdrv_drop_intermediate.
>
> If this function fails without making any changes, the new commit
> behavior is good. If it succeeds, we're also good. The problem is with
> intermediate or partial successes.
>
> If top has multiple parents (I think under normal circumstances it
> won't, but I'm not absolutely sure) and it fails to update their backing
> file references, it might partially succeed.
>
> I think commit's usage here is correct, but I think we might need to
> update bdrv_drop_intermediate to make it roll back changes if it
> experiences a partial failure to give all-or-nothing semantics.
Sure, that would be good.
> Thoughts?
We could start by calling bdrv_check_update_perm() on all parents before
doing any changes. Then the roll back would consist only of invoking
bdrv_abort_perm_update() and in theory reverting the
c->update_filename() changes.
In practice... How do we want to revert
Re: [Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
On 09/04/2018 02:46 PM, Jeff Cody wrote:
> On Tue, Sep 04, 2018 at 01:09:19PM -0400, John Snow wrote:
>> Use the component callbacks; prepare, abort, and clean.
>>
>> NB: prepare is only called when the job has not yet failed;
>> and abort can be called after prepare.
>>
>> complete -> prepare -> abort -> clean
>> complete -> abort -> clean
>>
>> Signed-off-by: John Snow
>> Reviewed-by: Max Reitz
>> ---
>> block/commit.c | 90
>> --
>> 1 file changed, 49 insertions(+), 41 deletions(-)
>>
>> diff --git a/block/commit.c b/block/commit.c
>> index b6e8969877..eb3941e545 100644
>> --- a/block/commit.c
>> +++ b/block/commit.c
>> @@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
>> BlockDriverState *commit_top_bs;
>> BlockBackend *top;
>> BlockBackend *base;
>> +BlockDriverState *base_bs;
>> BlockdevOnError on_error;
>> int base_flags;
>> char *backing_file_str;
>> @@ -68,61 +69,65 @@ static int coroutine_fn commit_populate(BlockBackend
>> *bs, BlockBackend *base,
>> return 0;
>> }
>>
>> -static void commit_exit(Job *job)
>> +static int commit_prepare(Job *job)
>> {
>> CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
>> -BlockJob *bjob = >common;
>> -BlockDriverState *top = blk_bs(s->top);
>> -BlockDriverState *base = blk_bs(s->base);
>> -BlockDriverState *commit_top_bs = s->commit_top_bs;
>> -bool remove_commit_top_bs = false;
>> -
>> -/* Make sure commit_top_bs and top stay around until
>> bdrv_replace_node() */
>> -bdrv_ref(top);
>> -bdrv_ref(commit_top_bs);
>>
>> /* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE before
>> * the normal backing chain can be restored. */
>> blk_unref(s->base);
>> +s->base = NULL;
>>
>> -if (!job_is_cancelled(job) && job->ret == 0) {
>> -/* success */
>> -job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
>> - s->backing_file_str);
>> -} else {
>> -/* XXX Can (or should) we somehow keep 'consistent read' blocked
>> even
>> - * after the failed/cancelled commit job is gone? If we already
>> wrote
>> - * something to base, the intermediate images aren't valid any
>> more. */
>> -remove_commit_top_bs = true;
>> +return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
>> + s->backing_file_str);
>> +}
>
> If we can go from prepare->abort->clean, then that means to me that every
> failure case of .prepare() can be resolved without permanent changes / data
> loss. Is this necessarily the case?
>
That'd be a requisite to make the job a transaction, but commit, mirror
and stream are not currently transactionable.
The way commit already works, for example, can leave the base and
intermediate images as unusable as standalone images. This refactoring
will not change that alone.
So it's not necessarily a problem, but it's something that would need to
be fixed if we ever wanted transaction support.
However, in talking on IRC we did realize that this patch does change
behavior...
Before:
If bdrv_drop_intermediate fails, we store the retcode but continue
cleaning up as if it didn't fail. i.e., we don't remove the commit job's
installed top_bs node.
After:
if bdrv_drop_intermediate fails, we return the failure retcode and
.abort gets called as a result, i.e. we will remove the commit job's
installed top_bs node in favor of the original top_bs node.
I think this behavior is an improvement, however it raises a question
about the nature of failures in bdrv_drop_intermediate.
If this function fails without making any changes, the new commit
behavior is good. If it succeeds, we're also good. The problem is with
intermediate or partial successes.
If top has multiple parents (I think under normal circumstances it
won't, but I'm not absolutely sure) and it fails to update their backing
file references, it might partially succeed.
I think commit's usage here is correct, but I think we might need to
update bdrv_drop_intermediate to make it roll back changes if it
experiences a partial failure to give all-or-nothing semantics.
Thoughts?
> From bdrv_drop_intermediate():
>
> QLIST_FOREACH_SAFE(c, >parents, next_parent, next) {
> /* Check whether we are allowed to switch c from top to base */
> GSList *ignore_children = g_slist_prepend(NULL, c);
> bdrv_check_update_perm(base, NULL, c->perm, c->shared_perm,
>ignore_children, _err);
> g_slist_free(ignore_children);
> if (local_err) {
> ret = -EPERM;
> error_report_err(local_err);
> goto exit;
> }
>
> /* If so, update the backing file path in the image file */
> if (c->role->update_filename) {
> ret = c->role->update_filename(c, base, backing_file_str,
>
Re: [Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
On Tue, Sep 04, 2018 at 01:09:19PM -0400, John Snow wrote:
> Use the component callbacks; prepare, abort, and clean.
>
> NB: prepare is only called when the job has not yet failed;
> and abort can be called after prepare.
>
> complete -> prepare -> abort -> clean
> complete -> abort -> clean
>
> Signed-off-by: John Snow
> Reviewed-by: Max Reitz
> ---
> block/commit.c | 90
> --
> 1 file changed, 49 insertions(+), 41 deletions(-)
>
> diff --git a/block/commit.c b/block/commit.c
> index b6e8969877..eb3941e545 100644
> --- a/block/commit.c
> +++ b/block/commit.c
> @@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
> BlockDriverState *commit_top_bs;
> BlockBackend *top;
> BlockBackend *base;
> +BlockDriverState *base_bs;
> BlockdevOnError on_error;
> int base_flags;
> char *backing_file_str;
> @@ -68,61 +69,65 @@ static int coroutine_fn commit_populate(BlockBackend *bs,
> BlockBackend *base,
> return 0;
> }
>
> -static void commit_exit(Job *job)
> +static int commit_prepare(Job *job)
> {
> CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
> -BlockJob *bjob = >common;
> -BlockDriverState *top = blk_bs(s->top);
> -BlockDriverState *base = blk_bs(s->base);
> -BlockDriverState *commit_top_bs = s->commit_top_bs;
> -bool remove_commit_top_bs = false;
> -
> -/* Make sure commit_top_bs and top stay around until bdrv_replace_node()
> */
> -bdrv_ref(top);
> -bdrv_ref(commit_top_bs);
>
> /* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE before
> * the normal backing chain can be restored. */
> blk_unref(s->base);
> +s->base = NULL;
>
> -if (!job_is_cancelled(job) && job->ret == 0) {
> -/* success */
> -job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
> - s->backing_file_str);
> -} else {
> -/* XXX Can (or should) we somehow keep 'consistent read' blocked even
> - * after the failed/cancelled commit job is gone? If we already wrote
> - * something to base, the intermediate images aren't valid any more.
> */
> -remove_commit_top_bs = true;
> +return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
> + s->backing_file_str);
> +}
If we can go from prepare->abort->clean, then that means to me that every
failure case of .prepare() can be resolved without permanent changes / data
loss. Is this necessarily the case?
>From bdrv_drop_intermediate():
QLIST_FOREACH_SAFE(c, >parents, next_parent, next) {
/* Check whether we are allowed to switch c from top to base */
GSList *ignore_children = g_slist_prepend(NULL, c);
bdrv_check_update_perm(base, NULL, c->perm, c->shared_perm,
ignore_children, _err);
g_slist_free(ignore_children);
if (local_err) {
ret = -EPERM;
error_report_err(local_err);
goto exit;
}
/* If so, update the backing file path in the image file */
if (c->role->update_filename) {
ret = c->role->update_filename(c, base, backing_file_str,
_err);
if (ret < 0) {
bdrv_abort_perm_update(base);
error_report_err(local_err);
goto exit;
}
}
[...]
}
We could fail this but still have modified an image file backing filenames,
right?
Or am I incorrect about the intention here, that abort() can always be clean?
-Jeff
> +
> +static void commit_abort(Job *job)
> +{
> +CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
> +BlockDriverState *top_bs = blk_bs(s->top);
> +
> +/* Make sure commit_top_bs and top stay around until bdrv_replace_node()
> */
> +bdrv_ref(top_bs);
> +bdrv_ref(s->commit_top_bs);
> +
> +if (s->base) {
> +blk_unref(s->base);
> }
>
> +/* free the blockers on the intermediate nodes so that bdrv_replace_nodes
> + * can succeed */
> +block_job_remove_all_bdrv(>common);
> +
> +/* If bdrv_drop_intermediate() failed (or was not invoked), remove the
> + * commit filter driver from the backing chain now. Do this as the final
> + * step so that the 'consistent read' permission can be granted.
> + *
> + * XXX Can (or should) we somehow keep 'consistent read' blocked even
> + * after the failed/cancelled commit job is gone? If we already wrote
> + * something to base, the intermediate images aren't valid any more. */
> +bdrv_child_try_set_perm(s->commit_top_bs->backing, 0, BLK_PERM_ALL,
> +_abort);
> +bdrv_replace_node(s->commit_top_bs, backing_bs(s->commit_top_bs),
> + _abort);
> +
> +bdrv_unref(s->commit_top_bs);
> +bdrv_unref(top_bs);
> +}
>
[Qemu-block] [PATCH v4 04/15] block/commit: refactor commit to use job callbacks
Use the component callbacks; prepare, abort, and clean.
NB: prepare is only called when the job has not yet failed;
and abort can be called after prepare.
complete -> prepare -> abort -> clean
complete -> abort -> clean
Signed-off-by: John Snow
Reviewed-by: Max Reitz
---
block/commit.c | 90 --
1 file changed, 49 insertions(+), 41 deletions(-)
diff --git a/block/commit.c b/block/commit.c
index b6e8969877..eb3941e545 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -36,6 +36,7 @@ typedef struct CommitBlockJob {
BlockDriverState *commit_top_bs;
BlockBackend *top;
BlockBackend *base;
+BlockDriverState *base_bs;
BlockdevOnError on_error;
int base_flags;
char *backing_file_str;
@@ -68,61 +69,65 @@ static int coroutine_fn commit_populate(BlockBackend *bs,
BlockBackend *base,
return 0;
}
-static void commit_exit(Job *job)
+static int commit_prepare(Job *job)
{
CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
-BlockJob *bjob = >common;
-BlockDriverState *top = blk_bs(s->top);
-BlockDriverState *base = blk_bs(s->base);
-BlockDriverState *commit_top_bs = s->commit_top_bs;
-bool remove_commit_top_bs = false;
-
-/* Make sure commit_top_bs and top stay around until bdrv_replace_node() */
-bdrv_ref(top);
-bdrv_ref(commit_top_bs);
/* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE before
* the normal backing chain can be restored. */
blk_unref(s->base);
+s->base = NULL;
-if (!job_is_cancelled(job) && job->ret == 0) {
-/* success */
-job->ret = bdrv_drop_intermediate(s->commit_top_bs, base,
- s->backing_file_str);
-} else {
-/* XXX Can (or should) we somehow keep 'consistent read' blocked even
- * after the failed/cancelled commit job is gone? If we already wrote
- * something to base, the intermediate images aren't valid any more. */
-remove_commit_top_bs = true;
+return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
+ s->backing_file_str);
+}
+
+static void commit_abort(Job *job)
+{
+CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
+BlockDriverState *top_bs = blk_bs(s->top);
+
+/* Make sure commit_top_bs and top stay around until bdrv_replace_node() */
+bdrv_ref(top_bs);
+bdrv_ref(s->commit_top_bs);
+
+if (s->base) {
+blk_unref(s->base);
}
+/* free the blockers on the intermediate nodes so that bdrv_replace_nodes
+ * can succeed */
+block_job_remove_all_bdrv(>common);
+
+/* If bdrv_drop_intermediate() failed (or was not invoked), remove the
+ * commit filter driver from the backing chain now. Do this as the final
+ * step so that the 'consistent read' permission can be granted.
+ *
+ * XXX Can (or should) we somehow keep 'consistent read' blocked even
+ * after the failed/cancelled commit job is gone? If we already wrote
+ * something to base, the intermediate images aren't valid any more. */
+bdrv_child_try_set_perm(s->commit_top_bs->backing, 0, BLK_PERM_ALL,
+_abort);
+bdrv_replace_node(s->commit_top_bs, backing_bs(s->commit_top_bs),
+ _abort);
+
+bdrv_unref(s->commit_top_bs);
+bdrv_unref(top_bs);
+}
+
+static void commit_clean(Job *job)
+{
+CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
+
/* restore base open flags here if appropriate (e.g., change the base back
* to r/o). These reopens do not need to be atomic, since we won't abort
* even on failure here */
-if (s->base_flags != bdrv_get_flags(base)) {
-bdrv_reopen(base, s->base_flags, NULL);
+if (s->base_flags != bdrv_get_flags(s->base_bs)) {
+bdrv_reopen(s->base_bs, s->base_flags, NULL);
}
+
g_free(s->backing_file_str);
blk_unref(s->top);
-
-/* If there is more than one reference to the job (e.g. if called from
- * job_finish_sync()), job_completed() won't free it and therefore the
- * blockers on the intermediate nodes remain. This would cause
- * bdrv_set_backing_hd() to fail. */
-block_job_remove_all_bdrv(bjob);
-
-/* If bdrv_drop_intermediate() didn't already do that, remove the commit
- * filter driver from the backing chain. Do this as the final step so that
- * the 'consistent read' permission can be granted. */
-if (remove_commit_top_bs) {
-bdrv_child_try_set_perm(commit_top_bs->backing, 0, BLK_PERM_ALL,
-_abort);
-bdrv_replace_node(commit_top_bs, backing_bs(commit_top_bs),
- _abort);
-}
-
-bdrv_unref(commit_top_bs);
-bdrv_unref(top);
}
static int coroutine_fn commit_run(Job *job, Error **errp)
@@ -211,7 +216,9 @@ static const BlockJobDriver
