friedkiwi/qemu
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

block/io: fallback to bounce buffer if BLKZEROOUT is not supported because of alignment

Browse Source 
Commit 5634622bcb ("file-posix: allow BLKZEROOUT with -t writeback")
enables the BLKZEROOUT ioctl when using 'writeback' cache, regressing
certain 'qemu-img convert' invocations, because of a pre-existing
issue. Namely, the BLKZEROOUT ioctl might fail with errno EINVAL when
the request is shorter than the block size of the block device.
Fallback to the bounce buffer, similar to when the ioctl is not
supported at all, rather than treating such an error as fatal.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3257
Resolves: https://bugzilla.proxmox.com/show_bug.cgi?id=7197
Cc: qemu-stable@nongnu.org
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20260105143416.737482-1-f.ebner@proxmox.com>
[Added TODO comment describing a larger fix that could be implemented in
the future.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Fiona Ebner
2026-01-05 15:29:55 +01:00
committed by Stefan Hajnoczi
parent 3390aba56c
commit b4e28c304b
1 changed files with 12 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

13
block/io.c
View File

@@ -1918,7 +1918,18 @@ bdrv_co_do_pwrite_zeroes(BlockDriverState *bs, int64_t offset, int64_t bytes,
assert(!bs->supported_zero_flags);
}
if (ret == -ENOTSUP && !(flags & BDRV_REQ_NO_FALLBACK)) {
/*
* TODO The ret == -EINVAL && num < alignment case is a workaround for
* when request_alignment is 1 on files with cache=writeback. The Linux
* ioctl(BLKZEROOUT) requires block alignment and will fail with
* EINVAL. The block layer should align the request to
* write_zeroes_alignment instead of trying the syscall, failing, and
* falling back to a bounce buffer. Doing that is not easy so for now
* we use a bounce buffer:
* https://lore.kernel.org/qemu-devel/20260109120837.2772961-1-f.ebner@proxmox.com/
*/
if ((ret == -ENOTSUP || (ret == -EINVAL && num < alignment)) &&
!(flags & BDRV_REQ_NO_FALLBACK)) {
/* Fall back to bounce buffer if write zeroes is unsupported */
BdrvRequestFlags write_flags = flags & ~BDRV_REQ_ZERO_WRITE;