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Re: [PATCH] block/block-copy: always align copied region to cluster size

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH] block/block-copy: always align copied region to cluster size
Date: Thu, 6 Aug 2020 18:42:57 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0

06.08.2020 16:57, Stefan Reiter wrote:
Since commit 42ac214406e0 (block/block-copy: refactor task creation)
block_copy_task_create calculates the area to be copied via
bdrv_dirty_bitmap_next_dirty_area, but that can return an unaligned byte
count if the backing image's last cluster end is not aligned to the

Hmm, I assume you mean not "backing image" but just "image"? Backing seems 
unrelated, the problem is just unaligned image

bitmap's granularity.

Always ALIGN_UP the resulting bytes value to satisfy block_copy_do_copy,
which requires the 'bytes' parameter to be aligned to cluster size.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

This causes backups with unaligned image sizes to fail on the last block in my
testing (e.g. a backup job with 4k cluster size fails on a drive with 4097

Ohh. Sorry for this :(

I think, we want this case covered by some iotest.. Are you going to add a 
test? Or I can do it..

Alternatively one could remove the
   assert(QEMU_IS_ALIGNED(bytes, s->cluster_size));
from block_copy_do_copy, but I'd wager that's there for a reason?

Looking at the code I think, that the reason is just a convention. It may be 
changed, but it will need an audit of the code. For me your fix looks the 
correct thing to do.

  block/block-copy.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/block/block-copy.c b/block/block-copy.c
index f7428a7c08..023cb03200 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -142,6 +142,8 @@ static BlockCopyTask *block_copy_task_create(BlockCopyState 
          return NULL;

For readability, I'd also add an assertion:

      assert(QEMU_IS_ALIGNED(offset, s->cluster_size));

+    bytes = QEMU_ALIGN_UP(bytes, s->cluster_size);
      /* region is dirty, so no existent tasks possible in it */
      assert(!find_conflicting_task(s, offset, bytes));

Thanks for fixing!
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Best regards,

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