On 19.04.19 12:23, Vladimir Sementsov-Ogievskiy wrote:
> 17.04.2019 19:22, Max Reitz wrote:
>> On 16.04.19 12:02, Vladimir Sementsov-Ogievskiy wrote:
>>> 10.04.2019 23:20, Max Reitz wrote:
>>>> What bs->file and bs->backing mean depends on the node. For filter
>>>> nodes, both signify a node that will eventually receive all R/W
>>>> accesses. For format nodes, bs->file contains metadata and data, and
>>>> bs->backing will not receive writes -- instead, writes are COWed to
>>>> bs->file. Usually.
>>>>
>>>> In any case, it is not trivial to guess what a child means exactly with
>>>> our currently limited form of expression. It is better to introduce
>>>> some functions that actually guarantee a meaning:
>>>>
>>>> - bdrv_filtered_cow_child() will return the child that receives requests
>>>> filtered through COW. That is, reads may or may not be forwarded
>>>> (depending on the overlay's allocation status), but writes never go to
>>>> this child.
>>>>
>>>> - bdrv_filtered_rw_child() will return the child that receives requests
>>>> filtered through some very plain process. Reads and writes issued to
>>>> the parent will go to the child as well (although timing, etc. may be
>>>> modified).
>>>>
>>>> - All drivers but quorum (but quorum is pretty opaque to the general
>>>> block layer anyway) always only have one of these children: All read
>>>> requests must be served from the filtered_rw_child (if it exists), so
>>>> if there was a filtered_cow_child in addition, it would not receive
>>>> any requests at all.
>>>> (The closest here is mirror, where all requests are passed on to the
>>>> source, but with write-blocking, write requests are "COWed" to the
>>>> target. But that just means that the target is a special child that
>>>> cannot be introspected by the generic block layer functions, and that
>>>> source is a filtered_rw_child.)
>>>> Therefore, we can also add bdrv_filtered_child() which returns that
>>>> one child (or NULL, if there is no filtered child).
>>>>
>>>> Also, many places in the current block layer should be skipping filters
>>>> (all filters or just the ones added implicitly, it depends) when going
>>>> through a block node chain. They do not do that currently, but this
>>>> patch makes them.
>>>>
>>>> One example for this is qemu-img map, which should skip filters and only
>>>> look at the COW elements in the graph. The change to iotest 204's
>>>> reference output shows how using blkdebug on top of a COW node used to
>>>> make qemu-img map disregard the rest of the backing chain, but with this
>>>> patch, the allocation in the base image is reported correctly.
>>>>
>>>> Furthermore, a note should be made that sometimes we do want to access
>>>> bs->backing directly. This is whenever the operation in question is not
>>>> about accessing the COW child, but the "backing" child, be it COW or
>>>> not. This is the case in functions such as bdrv_open_backing_file() or
>>>> whenever we have to deal with the special behavior of @backing as a
>>>> blockdev option, which is that it does not default to null like all
>>>> other child references do.
>>>>
>>>> Finally, the query functions (query-block and query-named-block-nodes)
>>>> are modified to return any filtered child under "backing", not just
>>>> bs->backing or COW children. This is so that filters do not interrupt
>>>> the reported backing chain. This changes the output of iotest 184, as
>>>> the throttled node now appears as a backing child.
>>>>
>>>> Signed-off-by: Max Reitz <mre...@redhat.com>
>>>> ---
>>>> qapi/block-core.json | 4 +
>>>> include/block/block.h | 1 +
>>>> include/block/block_int.h | 40 +++++--
>>>> block.c | 210 +++++++++++++++++++++++++++------
>>>> block/backup.c | 8 +-
>>>> block/block-backend.c | 16 ++-
>>>> block/commit.c | 33 +++---
>>>> block/io.c | 45 ++++---
>>>> block/mirror.c | 21 ++--
>>>> block/qapi.c | 30 +++--
>>>> block/stream.c | 13 +-
>>>> blockdev.c | 88 +++++++++++---
>>>> migration/block-dirty-bitmap.c | 4 +-
>>>> nbd/server.c | 6 +-
>>>> qemu-img.c | 29 ++---
>>>> tests/qemu-iotests/184.out | 7 +-
>>>> tests/qemu-iotests/204.out | 1 +
>>>> 17 files changed, 411 insertions(+), 145 deletions(-)
>>>
>>> really huge... didn't you consider conversion file-by-file?
>>
>> Frankly, no, I just didn’t consider it.
>>
>> Hm. I don’t know, 30-patch series always look so frightening.
>>
>>>> diff --git a/block.c b/block.c
>>>> index 16615bc876..e8f6febda0 100644
>>>> --- a/block.c
>>>> +++ b/block.c
>>>
>>> [..]
>>>
>>>>
>>>> @@ -3467,14 +3469,17 @@ static int
>>>> bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
>>>> /*
>>>> * Find the "actual" backing file by skipping all links that point
>>>> * to an implicit node, if any (e.g. a commit filter node).
>>>> + * We cannot use any of the bdrv_skip_*() functions here because
>>>> + * those return the first explicit node, while we are looking for
>>>> + * its overlay here.
>>>> */
>>>> overlay_bs = bs;
>>>> - while (backing_bs(overlay_bs) && backing_bs(overlay_bs)->implicit) {
>>>> - overlay_bs = backing_bs(overlay_bs);
>>>> + while (overlay_bs->backing && bdrv_filtered_bs(overlay_bs)->implicit)
>>>> {
>>>
>>> So, you don't want to skip implicit filters with 'file' child? Then, why
>>> not to use
>>> child_bs(overlay_bs->backing), like in following if condition?
>>
>> I think it was an artifact of writing the patch. I started with
>> bdrv_filtered_bs() and then realized this depends on ->backing,
>> actually. There was no functional difference so I left it as it was.
>>
>> But you’re right, it is more clear to use child_bs(overlay_bs->backing)
>> isntead.
>>
>>> Could we instead make backing-based filters equal to file-based, to make it
>>> possible
>>> to use file-based filters in backing-chain related scenarios (like upcoming
>>> copy-on-read
>>> filter for stream)? So, to expand backing-chain concept to include filters
>>> with file child?
>>
>> If I understand you correctly, that’s basically the purpose of this
>> series and especially this patch here. As far as it is possible and
>> reasonable, I want filters that use bs->backing and bs->file behave the
>> same.
>>
>> However, there are cases where this is not possible and
>> bdrv_reopen_parse_backing() is one such case. bs->backing and bs->file
>> correspond to QAPI names, namely 'backing' and 'file'. If that
>> distinction was already visible to the user, we cannot change it now.
>>
>> We definitely cannot make file-based filters use bs->backing now because
>> you can create them over QAPI and they use 'file' as their child name.
>> Can we make backing-based filters use bs->file? Seems more likely,
>> because all of them are implicit nodes, so the user usually doesn’t see
>> them. But usually isn’t always; they do become user-visible once the
>> user specifies a node-name for mirror or commit.
>>
>> I found it more reasonable to introduce new functions that explicitly
>> express what kind of child they expect and then apply them everywhere as
>> I saw fit, instead of making the mirror/commit filter drivers use
>> bs->file and hope it works; not least because I’d still have to go
>> through the whole block layer and check every instance of bs->backing to
>> see whether it really needs bs->backing or whether it should use either
>> of bs->backing or bs->file.
>>
>>>> + overlay_bs = bdrv_filtered_bs(overlay_bs);
>>>> }
>>>>
>>>> /* If we want to replace the backing file we need some extra checks
>>>> */
>>>> - if (new_backing_bs != backing_bs(overlay_bs)) {
>>>> + if (new_backing_bs != child_bs(overlay_bs->backing)) { > /*
>>>> Check for implicit nodes between bs and its backing file */
>>>> if (bs != overlay_bs) {
>>>> error_setg(errp, "Cannot change backing link if '%s' has "
>>>
>>> [..]
>>>
>>>> @@ -4203,8 +4208,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>>>> BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
>>>> BlockDriverState *bs)
>>>> {
>>>> - while (active && bs != backing_bs(active)) {
>>>> - active = backing_bs(active);
>>>> + while (active && bs != bdrv_filtered_bs(active)) {
>>>
>>> hmm and here you actually support backing-chain with file-child-based
>>> filters in it..
>>
>> Yes, because this is not about the QAPI 'backing' link. This function
>> should continue to work even if there are filters in the backing chain.
>
> this is a generic function to find overlay in backing chain and it may be
> used from different places,
> for example it is used in Andrey's series about filter for block-stream.
Well, all places that use it accept backing chains with filters inside
of them.
> It is used from qmp_block_commit, isn't it about QAPI?
By "QAPI 'backing' link" I mean the user-visible block graph. Hm. I
wrote in my other mail that you could use query-named-block-nodes to see
that graph; apparently you can’t. So besides x-debug-query-block-graph,
we still don’t have any facility to query the block graph? I don’t know
what to say.
Anyway, you can still construct the graph with blockdev-add, so it is
user-visible. And in that block graph, there is a 'backing' link, and
there is a 'file' link -- this is what I mean with "QAPI link".
We have commands that are abstract and don’t work on specific graph
links. For instance, block-commit commits across a backing chain, so it
doesn’t matter whether the graph link is called 'backing' or whatever,
what is important is that it’s a COW link. But we should also ignore
filters on the way, so this patch makes block-commit and others use
those more abstract child access functions.
But whenever it is about exactly the "file" or the "backing" link, we
have to use bs->file and bs->backing, respectively. That's just how it
currently is.
>>>> + active = bdrv_filtered_bs(active);
>>>> }
>>>>
>>>> return active;
>>>> @@ -4226,11 +4231,11 @@ bool bdrv_is_backing_chain_frozen(BlockDriverState
>>>> *bs, BlockDriverState *base,
>>>> {
>>>> BlockDriverState *i;
>>>>
>>>> - for (i = bs; i != base; i = backing_bs(i)) {
>>>> + for (i = bs; i != base; i = child_bs(i->backing)) {
>>>
>>> and here don't..
>>
>> Yes, because this function is about the QAPI 'backing' link.
>
> And this again a generic thing, that may be used in same places as
> bdrv_find_overlay,
But it isn’t.
> and it is used in series about block-stream filter too. So, for further
> developments
> we'll have to keep in mind all these differences between generic block layer
> functions,
> which supports .file children inside backing chain and which are not...
I was wrong about bdrv_is_backing_chain_frozen(), if that helps (as I
wrote in my other (previous) mail).
But for example bdrv_set_backing_hd() always has to use bs->backing,
because that’s what it’s about (and I do change its descriptive comment
to reflect that, so you don’t need to keep it in mind). Same for
bdrv_open_backing_file().
Hm, what other cases are there...
bdrv_reopen_parse_backing(): Fundamentally, this too is about the
user-visible "backing" link (as specified through x-blockdev-reopen).
But the loop it contains is more difficult to translate than I had
thought. At some point, there needs to be a bs->backing link, because
that is what this function is about, but it should also skip all
implicit filters in the way, I think. So e.g. this should be recognized:
bs ---backing--> COR ---file--> base
@overlay_bs should be COR, I think...? I mean, as long as COR is an
implicit node. So the loop really should use bdrv_filtered_bs()
everywhere, and then the same afterwards. I think that we should also
ensure that @bs can support a ->backing child, but how would I check
that? Maybe it’s safe to just omit such a check...
But then another issue comes in: The link to replace (in the above case
from "COR" to "base") is no longer necessarily a backing link. So
bdrv_reopen_commit() has to be capable of replacing both bs->backing and
bs->file.
Actually, how does bdrv_reopen_commit() handle implicit nodes at all?
bdrv_reopen_parse_backing() just sets reopen_state->replace_backing_bs
and ->new_backing_bs. It doesn’t communicate anything about overlay_bs.
bdrv_reopen_commit() then asserts that !bs->backing->bs->implicit and
replaces bs->backing. So it seems to just fail on the implicit nodes
that bdrv_reopen_parse_backing() took care to skip...
OK, what else... bdrv_reopen_prepare() checks
reopen_state->bs->backing, which I claim is correct because while there
may be implicit filters in the chain, the first link has to be a
->backing link.
bdrv_backing_overridden() has to query bs->backing because this function
is used when it is about a specific characteristic of the backing link:
There is a non-null default (given by the image header), so if the
current bs->backing matches this default, you do not have to specify the
backing filename in either blockdev-add or a filename. Same in
bdrv_refresh_filename().
I hope that was all...?
Max
signature.asc
Description: OpenPGP digital signature
