Re: [PATCH] block/file-posix: Consider discard flag when opening

[Kevin Wolf <kwolf@redhat.com>]

Am 18.06.2024 um 23:24 hat Nir Soffer geschrieben:
> Set has_discard only when BDRV_O_UNMAP is not set. With this users that
> want to keep their images fully allocated can disable hole punching
> when writing zeros or discarding using:
> 
>    -drive file=thick.img,discard=off
> 
> This change is not entirely correct since it changes the default discard
> behavior.  Previously we always allowed punching holes, but now you have
> must use discard=unmap|on to enable it. We probably need to add the
> BDDR_O_UNMAP flag by default.
> 
> make check still works, so maybe we don't have tests for sparsifying
> images, or maybe you need to run special tests that do not run by
> default. We needs tests for keeping images non-sparse.
> 
> Signed-off-by: Nir Soffer <nsoffer@redhat.com>

So first of all, I agree with you that this patch is wrong. ;-)

At first, I failed to understand the problem this is trying to solve. I
put a debug message in handle_aiocb_discard() and tried with which
options it triggers. [1] To me, this looked exactly like it should be.
We only try to discard blocks when discard=unmap is given as an option.

That leaves the case of write_zeroes. And while at the first sight, the
code looked good, we do seem to have a problem there and it tried to
unmap even with discard=off.

>  block/file-posix.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index be25e35ff6..acac2abadc 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -738,11 +738,11 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
>          ret = -EINVAL;
>          goto fail;
>      }
>  #endif /* !defined(CONFIG_LINUX_IO_URING) */
>  
> -    s->has_discard = true;
> +    s->has_discard = !!(bdrv_flags & BDRV_O_UNMAP);
>      s->has_write_zeroes = true;
>  
>      if (fstat(s->fd, &st) < 0) {
>          ret = -errno;
>          error_setg_errno(errp, errno, "Could not stat file");

s->has_discard is about what the host supports, not about the semantics
of the QEMU block node. So this doesn't feel right to me.

So for the buggy case, write_zeroes, bdrv_co_pwrite_zeroes() has code
that considers the case and clears the ~BDRV_REQ_MAY_UNMAP flags:

    if (!(child->bs->open_flags & BDRV_O_UNMAP)) {
        flags &= ~BDRV_REQ_MAY_UNMAP;
    }

But it turns out that we don't necessarily even go through this function
for the top node which has discard=off, so it can't take effect:

(gdb) bt
#0  0x00007ffff4f2f144 in __pthread_kill_implementation () at /lib64/libc.so.6
#1  0x00007ffff4ed765e in raise () at /lib64/libc.so.6
#2  0x00007ffff4ebf902 in abort () at /lib64/libc.so.6
#3  0x000055555615aff0 in raw_do_pwrite_zeroes (bs=0x555557f4bcf0, offset=0, bytes=1048576, flags=BDRV_REQ_MAY_UNMAP, blkdev=false) at ../block/file-posix.c:3643
#4  0x000055555615557e in raw_co_pwrite_zeroes (bs=0x555557f4bcf0, offset=0, bytes=1048576, flags=BDRV_REQ_MAY_UNMAP) at ../block/file-posix.c:3655
#5  0x00005555560cde2a in bdrv_co_do_pwrite_zeroes (bs=0x555557f4bcf0, offset=0, bytes=1048576, flags=6) at ../block/io.c:1901
#6  0x00005555560c72f9 in bdrv_aligned_pwritev (child=0x555557f51460, req=0x7fffed5ff800, offset=0, bytes=1048576, align=1, qiov=0x0, qiov_offset=0, flags=6) at ../block/io.c:2100
#7  0x00005555560c6b41 in bdrv_co_do_zero_pwritev (child=0x555557f51460, offset=0, bytes=1048576, flags=6, req=0x7fffed5ff800) at ../block/io.c:2183
#8  0x00005555560c6647 in bdrv_co_pwritev_part (child=0x555557f51460, offset=0, bytes=1048576, qiov=0x0, qiov_offset=0, flags=6) at ../block/io.c:2283
#9  0x00005555560c634f in bdrv_co_pwritev (child=0x555557f51460, offset=0, bytes=1048576, qiov=0x0, flags=6) at ../block/io.c:2216
#10 0x00005555560c75b5 in bdrv_co_pwrite_zeroes (child=0x555557f51460, offset=0, bytes=1048576, flags=BDRV_REQ_MAY_UNMAP) at ../block/io.c:2322
#11 0x0000555556117d24 in raw_co_pwrite_zeroes (bs=0x555557f44980, offset=0, bytes=1048576, flags=BDRV_REQ_MAY_UNMAP) at ../block/raw-format.c:307
#12 0x00005555560cde2a in bdrv_co_do_pwrite_zeroes (bs=0x555557f44980, offset=0, bytes=1048576, flags=6) at ../block/io.c:1901
#13 0x00005555560c72f9 in bdrv_aligned_pwritev (child=0x555557f513f0, req=0x7fffed5ffd90, offset=0, bytes=1048576, align=1, qiov=0x0, qiov_offset=0, flags=6) at ../block/io.c:2100
#14 0x00005555560c6b41 in bdrv_co_do_zero_pwritev (child=0x555557f513f0, offset=0, bytes=1048576, flags=6, req=0x7fffed5ffd90) at ../block/io.c:2183
#15 0x00005555560c6647 in bdrv_co_pwritev_part (child=0x555557f513f0, offset=0, bytes=1048576, qiov=0x0, qiov_offset=0, flags=6) at ../block/io.c:2283
#16 0x00005555560ad741 in blk_co_do_pwritev_part (blk=0x555557f51660, offset=0, bytes=1048576, qiov=0x0, qiov_offset=0, flags=6) at ../block/block-backend.c:1425
#17 0x00005555560ad5f2 in blk_co_pwritev_part (blk=0x555557f51660, offset=0, bytes=1048576, qiov=0x0, qiov_offset=0, flags=6) at ../block/block-backend.c:1440
#18 0x00005555560ad8cf in blk_co_pwritev (blk=0x555557f51660, offset=0, bytes=1048576, qiov=0x0, flags=6) at ../block/block-backend.c:1462
#19 0x00005555560b0f79 in blk_co_pwrite_zeroes (blk=0x555557f51660, offset=0, bytes=1048576, flags=6) at ../block/block-backend.c:2590
#20 0x000055555606d240 in blk_co_pwrite_zeroes_entry (opaque=0x7fffffffbc18) at block/block-gen.c:2162
#21 0x00005555562c36ba in coroutine_trampoline (i0=1475685216, i1=21845) at ../util/coroutine-ucontext.c:175
#22 0x00007ffff4ef1190 in ??? () at /lib64/libc.so.6
#23 0x00007fffffffc010 in ??? ()
#24 0x0000000000000000 in ??? ()

I haven't checked the details yet, but my first impression is that this
check should probably move to bdrv_co_do_pwrite_zeroes().

Kevin


[1] Tests I did:

# For discard

$ echo -e 'qemu-io none0 "discard 0 1M"\nquit' | ./qemu-system-x86_64 -drive if=none,file=/tmp/test.raw,format=raw -monitor stdio
QEMU 9.0.50 monitor - type 'help' for more information
(qemu) qemu-io none0 "discard 0 1M"
discard 1048576/1048576 bytes at offset 0
1 MiB, 1 ops; 00.00 sec (396.171 GiB/sec and 405679.5132 ops/sec)
(qemu) quit

$ echo -e 'qemu-io none0 "discard 0 1M"\nquit' | ./qemu-system-x86_64 -drive if=none,file=/tmp/test.raw,format=raw,discard=unmap -monitor stdio
QEMU 9.0.50 monitor - type 'help' for more information
(qemu) qemu-io none0 "discard 0 1M"
handle_aiocb_discard
discard 1048576/1048576 bytes at offset 0
1 MiB, 1 ops; 00.00 sec (6.623 GiB/sec and 6782.2820 ops/sec)
(qemu) quit

$ echo -e 'qemu-io none0 "discard 0 1M"\nquit' | ./qemu-system-x86_64 -drive if=none,file=/tmp/test.raw,format=raw,discard=off -monitor stdio
QEMU 9.0.50 monitor - type 'help' for more information
(qemu) qemu-io none0 "discard 0 1M"
discard 1048576/1048576 bytes at offset 0
1 MiB, 1 ops; 00.00 sec (375.168 GiB/sec and 384172.1091 ops/sec)
(qemu) quit

# For write_zeroes

$ echo -e 'qemu-io none0 "write -zu 0 1M"\nquit' | ./qemu-system-x86_64 -drive if=none,file=/tmp/test.raw,format=raw,discard=unmap -monitor stdio
QEMU 9.0.50 monitor - type 'help' for more information
(qemu) qemu-io none0 "write -zu 0 1M"
handle_aiocb_write_zeroes_unmap
wrote 1048576/1048576 bytes at offset 0
1 MiB, 1 ops; 00.00 sec (4.943 GiB/sec and 5061.5997 ops/sec)
(qemu) quit
$ echo -e 'qemu-io none0 "write -zu 0 1M"\nquit' | ./qemu-system-x86_64 -drive if=none,file=/tmp/test.raw,format=raw,discard=off -monitor stdio
QEMU 9.0.50 monitor - type 'help' for more information
(qemu) qemu-io none0 "write -zu 0 1M"
handle_aiocb_write_zeroes_unmap
wrote 1048576/1048576 bytes at offset 0
1 MiB, 1 ops; 00.00 sec (7.208 GiB/sec and 7381.4357 ops/sec)
(qemu) quit