On 12.03.21 16:24, Vladimir Sementsov-Ogievskiy wrote:
12.03.2021 18:10, Max Reitz wrote:
On 12.03.21 13:46, Vladimir Sementsov-Ogievskiy wrote:
12.03.2021 15:32, Vladimir Sementsov-Ogievskiy wrote:
12.03.2021 14:17, Max Reitz wrote:
On 12.03.21 10:09, Vladimir Sementsov-Ogievskiy wrote:
11.03.2021 22:58, Max Reitz wrote:
On 05.03.21 18:35, Vladimir Sementsov-Ogievskiy wrote:
There is a bug in qcow2: host cluster can be discarded (refcount
becomes 0) and reused during data write. In this case data write
may
[..]
@@ -885,6 +1019,13 @@ static int QEMU_WARN_UNUSED_RESULT
update_refcount(BlockDriverState *bs,
if (refcount == 0) {
void *table;
+ Qcow2InFlightRefcount *infl = find_infl_wr(s,
cluster_index);
+
+ if (infl) {
+ infl->refcount_zero = true;
+ infl->type = type;
+ continue;
+ }
I don’t understand what this is supposed to do exactly. It seems
like it wants to keep metadata structures in the cache that are
still in use (because dropping them from the caches is what
happens next), but users of metadata structures won’t set
in-flight counters for those metadata structures, will they?
Don't follow.
We want the code in "if (refcount == 0)" to be triggered only when
full reference count of the host cluster becomes 0, including
inflight-write-cnt. So, if at this point inflight-write-cnt is not
0, we postpone freeing the host cluster, it will be done later
from "slow path" in update_inflight_write_cnt().
But the code under “if (refcount == 0)” doesn’t free anything, does
it? All I can see is code to remove metadata structures from the
metadata caches (if the discarded cluster was an L2 table or a
refblock), and finally the discard on the underlying file. I don’t
see how that protocol-level discard has anything to do with our
problem, though.
Hmm. Still, if we do this discard, and then our in-flight write,
we'll have data instead of a hole. Not a big deal, but seems better
to postpone discard.
On the other hand, clearing caches is OK, as its related only to
qcow2-refcount, not to inflight-write-cnt
As far as I understand, the freeing happens immediately above the
“if (refcount == 0)” block by s->set_refcount() setting the
refcount to 0. (including updating s->free_cluster_index if the
refcount is 0).
Hmm.. And that (setting s->free_cluster_index) what I should
actually prevent until total reference count becomes zero.
And about s->set_refcount(): it only update a refcount itself, and
don't free anything.
So, it is more correct like this:
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 464d133368..1da282446d 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -1012,21 +1012,12 @@ static int QEMU_WARN_UNUSED_RESULT
update_refcount(BlockDriverState *bs,
} else {
refcount += addend;
}
- if (refcount == 0 && cluster_index < s->free_cluster_index) {
- s->free_cluster_index = cluster_index;
- }
s->set_refcount(refcount_block, block_index, refcount);
if (refcount == 0) {
void *table;
Qcow2InFlightRefcount *infl = find_infl_wr(s,
cluster_index);
- if (infl) {
- infl->refcount_zero = true;
- infl->type = type;
- continue;
- }
-
table =
qcow2_cache_is_table_offset(s->refcount_block_cache,
offset);
if (table != NULL) {
@@ -1040,6 +1031,16 @@ static int QEMU_WARN_UNUSED_RESULT
update_refcount(BlockDriverState *bs,
qcow2_cache_discard(s->l2_table_cache, table);
}
+ if (infl) {
+ infl->refcount_zero = true;
+ infl->type = type;
+ continue;
+ }
+
+ if (cluster_index < s->free_cluster_index) {
+ s->free_cluster_index = cluster_index;
+ }
+
if (s->discard_passthrough[type]) {
update_refcount_discard(bs, cluster_offset,
s->cluster_size);
}
I don’t think I like using s->free_cluster_index as a protection
against allocating something before it.
Hmm, I just propose not to update it, if refcount reached 0 but we still
have inflight writes.
First, it comes back the problem I just described in my mail from
15:58 GMT+1, which is that you’re changing the definition of what a
free cluster is. With this proposal, you’re proposing yet a new
definition: A free cluster is anything with refcount == 0 after
free_cluster_index.
I think that free cluster is anything with refcount = 0 and
inflight-write-cnt = 0.
Then, as I said in my other mail, update_refcount() just cannot free any
cluster. So changes to that function can’t be justified by preventing
it from freeing clusters.
You need to clearly define what it is that update_refcount() should or
shouldn’t do, and then we have to think about whether when all writes
have settled, we really have to invoke qcow2_update_cluster_refcount()
or whether we should do the small outstanding changes just directly in
update_inflight_write_cnt().
I think this needs to be more formalized, or it doesn’t make sense.
For example, say we do define a free cluster to be refcount (RC) = 0 and
inflight-write-cnt (IFWC) = 0. Then everything that is done to a
cluster because it is considered being freed right now because its RC
drops to 0 must probably be changed to only be done if also its IFWC is
0. For example, we should only discard host clusters on the protocol
layer if a cluster becomes free. update_refcount() will no longer be
able to free clusters with IFWC > 0, so it must never issue a
protocol-level discard for them. And, yes, it also shouldn’t adjust
first_free_cluster_index, as you propose here. (But you didn’t explain
why, and it seems like it was just intuition to you instead of looking
at it more formally.)
Instead, for clusters with RC = 0 and IFWC > 0,
update_inflight_write_cnt() will take on the role of freeing them. So
now that function must adjust first_free_cluster_index and issue the
protocol-level discard for such clusters.
I suppose in practice we could invoke qcow2_update_cluster_refcount()
with -0, as you do, because now the cluster has RC = 0 and IFWC = 0, so
now that function will be capable of freeing it. But to me, that just
looks like a bit of abuse.
I suppose we could create a new function qcow2_cluster_freed() where we
collect everything that needs to be done once a cluster is considered
freed (which so far was whenever its RC dropped to 0, which only happens
in update_refcount(); and then will be whenever its RC and its IFWC drop
to 0, which can happen in either update_refcount() or
update_inflight_write_cnt()). What would belong in there is discarding
the cluster on the protocol level, and adjusting
first_free_cluster_index. (Perhaps more, I don’t know.) With such a
function, it would seem clear to me that there is no need to invoke
qcow2_update_cluster_refcount() just to get precisely that effect.
(The alternative would be to keep RC == 0 the definition of a freed
cluster. Then we’d have to postpone the s->set_refcount() in
update_refcount(), and update the refcount again in
update_inflight_write_cnt(), but invoking
qcow2_update_cluster_refcount(). We wouldn’t need to change the
allocation functions.
I’m not saying that alternative is better – I don’t think it is, I think
you’re right that the definition of a freed cluster should be changed.
I’m just presenting it in contrast, to show when it would make sense to
call qcow2_update_cluster_refcount().)
And free_cluster_index is a hint where start to
search for such cluster.
Now looking only at the allocation functions, it may look like that
kind of is the definition already. But I don’t think that was the
intention when free_cluster_index was introduced, so we’d have to
check every place that sets free_cluster_index, to see whether it
adheres to this definition.
And I think it’s clear that there is a place that won’t adhere to this
definition, and that is this very place here, in update_refcount().
Say free_cluster_index is 42. Then you free cluster 39, but there is
a write to it, so free_cluster_index isn’t update. Then you free
cluster 38, and there are writes to that cluster, so
free_cluster_index is updated to 38. Suddenly, 39 is free to be
allocated, too.
Why? 39 is protected by inflight-cnt, and we do has_infl_wr() check
together with refcount==0 check when allocate clusters.
I was (wrongly) assuming that with this change you’d drop the check in
the allocation functions.
Max
(The precise problem is that with this new definition decreasing
free_cluster_index suddenly has the power to free any cluster between
its new and all value. With the old definition, changing
free_cluster_index would never free any cluster. So when you decrease
free_cluster_index, you suddenly have to be sure that all clusters
between the new and old value that have refcount 0 are indeed to be
considered free.)
Max