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file-posix: allow BLKZEROOUT with -t writeback

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The Linux BLKZEROOUT ioctl is only invoked when BDRV_O_NOCACHE is set
because old kernels did not invalidate the page cache. In that case
mixing BLKZEROOUT with buffered I/O could lead to corruption.

However, Linux 4.9 commit 22dd6d356628 ("block: invalidate the page
cache when issuing BLKZEROOUT") made BLKZEROOUT coherent with the page
cache.

I have checked that Linux 4.9+ kernels are shipped at least as far back
as Debian 10 (buster), openSUSE Leap 15.2, and RHEL/CentOS 8.

Use BLKZEROOUT with buffered I/O, mostly so `qemu-img ... -t
writeback` can offload write zeroes.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250417211053.98700-1-stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2025-04-17 17:10:53 -04:00 • committed by Kevin Wolf
parent ff2ab634e4
commit 5634622bcb
1 changed files with 0 additions and 11 deletions
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11
block/file-posix.c
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@ -785,17 +785,6 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
}
#endif
if (S_ISBLK(st.st_mode)) {
#ifdef __linux__
/* On Linux 3.10, BLKDISCARD leaves stale data in the page cache. Do
* not rely on the contents of discarded blocks unless using O_DIRECT.
* Same for BLKZEROOUT.
*/
if (!(bs->open_flags & BDRV_O_NOCACHE)) {
s->has_write_zeroes = false;
}
#endif
}
#ifdef __FreeBSD__
if (S_ISCHR(st.st_mode)) {
/*