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Re: [Qemu-devel] [PATCH for 2.7 v3 1/1] qcow2: improve qcow2_co_write_ze

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [PATCH for 2.7 v3 1/1] qcow2: improve qcow2_co_write_zeroes()
Date: Fri, 13 May 2016 19:09:39 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

On 05/12/2016 01:37 PM, Kevin Wolf wrote:
Am 12.05.2016 um 11:00 hat Denis V. Lunev geschrieben:
On 05/11/2016 02:28 PM, Kevin Wolf wrote:
Am 11.05.2016 um 09:00 hat Denis V. Lunev geschrieben:
There is a possibility that qcow2_co_write_zeroes() will be called
with the partial block. This could be synthetically triggered with
     qemu-io -c "write -z 32k 4k"
and can happen in the real life in qemu-nbd. The latter happens under
the following conditions:
     (1) qemu-nbd is started with --detect-zeroes=on and is connected to the
         kernel NBD client
     (2) third party program opens kernel NBD device with O_DIRECT
     (3) third party program performs write operation with memory buffer
         not aligned to the page
In this case qcow2_co_write_zeroes() is unable to perform the operation
and mark entire cluster as zeroed and returns ENOTSUP. Thus the caller
switches to non-optimized version and writes real zeroes to the disk.

The patch creates a shortcut. If the block is read as zeroes, f.e. if
it is unallocated, the request is extended to cover full block.
User-visible situation with this block is not changed. Before the patch
the block is filled in the image with real zeroes. After that patch the
block is marked as zeroed in metadata. Thus any subsequent changes in
backing store chain are not affected.

Kevin, thank you for a cool suggestion.

Signed-off-by: Denis V. Lunev <address@hidden>
Reviewed-by: Roman Kagan <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Max Reitz <address@hidden>
Changes from v2:
- checked head/tail clusters separately (one can be zeroed, one unallocated)
- fixed range calculations
- fixed race when the block can become used just after the check
- fixed zero cluster detection
- minor tweaks in the description

Changes from v1:
- description rewritten completely
- new approach suggested by Kevin is implemented

  block/qcow2.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++------
  1 file changed, 59 insertions(+), 6 deletions(-)
oops, the patch gets committed... that is unexpected but great ;)

diff --git a/block/qcow2.c b/block/qcow2.c
index 470734b..c2474c1 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2411,21 +2411,74 @@ finish:
      return ret;
+static bool is_zero_cluster(BlockDriverState *bs, int64_t start)
+    BDRVQcow2State *s = bs->opaque;
+    int nr;
+    BlockDriverState *file;
+    int64_t res = bdrv_get_block_status_above(bs, NULL, start,
+                                              s->cluster_sectors, &nr, &file);
+    return res >= 0 && ((res & BDRV_BLOCK_ZERO) || !(res & BDRV_BLOCK_DATA));
Why did you add the !(res & BDRV_BLOCK_DATA) condition? This means that
all unallocated clusters return true, even if the backing file contains
non-zero data for them.
this is correct. From my POW this means that this area is unallocated
in the entire backing chain and thus it will be read as zeroes. Thus
we could cover it with zeroes.
You're right that I made a mistake, I was thinking of the non-recursive

However, I still think that we may not assume that !BDRV_BLOCK_DATA
means zero data, even though that affects only more obscure cases. We
have bdrv_unallocated_blocks_are_zero() to check whether the assumption
is true. However, bdrv_co_get_block_status() already checks this
internally and sets BDRV_BLOCK_ZERO in this case, so just checking
BDRV_BLOCK_ZERO in qcow2 should be good.

Did you find a case where you got !DATA, but not ZERO, and assuming
zeroes was valid? If so, we may need to fix bdrv_co_get_block_status().
actually we may have the following case (artificial)!:
- assuming we do not have bdrv_has_zero_init in backing store
- and qcow2 on top of this file
- reading from unallocated block should return 0 (no data in both places), qcow2
  layer will return 0

It looks like we will have this situation.


Hm, I see:

     if (bs->bl.write_zeroes_alignment
         && num > bs->bl.write_zeroes_alignment) {

Removing the second part should fix this, i.e. it would split a request
into two unaligned halves even if there is no aligned "bulk" in the

I think it would match my expectations better, but maybe that's just me.
What do you think?
actually the code here will not be significantly better (I presume),
but I'll make a try

+            cl_end = sector_num + nb_sectors - s->cluster_sectors;
+            if (!is_zero_cluster(bs, cl_end)) {
+                return -ENOTSUP;
+            }
+        }
+        qemu_co_mutex_lock(&s->lock);
+        /* We can have new write after previous check */
+        if (!is_zero_cluster_top_locked(bs, sector_num) ||
+                (cl_end > 0 && !is_zero_cluster_top_locked(bs, cl_end))) {
+            qemu_co_mutex_unlock(&s->lock);
+            return -ENOTSUP;
+        }
Just lock the mutex before the check, the possible optimisation for the
emulation case (which is slow anyway) isn't worth the additional code
bdrv_get_block_status_above(bs) takes s->lock inside. This lock is not
recursive thus the code will hang. This is the problem trying to be
addressed with this split of checks.

May be we could make the lock recursive...
Maybe your version is no far from the best we can do then. It deserves a
comment, though, because it's not completely obvious.

The other option that we have and that looks reasonable enough to me is
checking is_zero_cluster_top_locked() first and only if that returns
false, we check the block status of the backing chain, starting at
bs->backing->bs. This way we would bypass the recursive call and could
take the lock from the beginning. If we go that way, it deserves a
comment as well.

OK. I'll send at least improved comments and (may be)
removal of "&& num > bs->bl.write_zeroes_alignment"
as follow up.

thank you for ideas ;)


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