Re: [Qemu-devel] [PATCH 1/2] block/file-posix: Unaligned O_DIRECT block-status

[Eric Blake]

On 5/14/19 4:42 PM, Max Reitz wrote:
> Currently, qemu crashes whenever someone queries the block status of an
> unaligned image tail of an O_DIRECT image:
> $ echo > foo
> $ qemu-img map --image-opts driver=file,filename=foo,cache.direct=on
> Offset          Length          Mapped to       File
> qemu-img: block/io.c:2093: bdrv_co_block_status: Assertion `*pnum &&
> QEMU_IS_ALIGNED(*pnum, align) && align > offset - aligned_offset'
> failed.
> 
> This is because bdrv_co_block_status() checks that the result returned
> by the driver's implementation is aligned to the request_alignment, but
> file-posix can fail to do so, which is actually mentioned in a comment
> there: "[...] possibly including a partial sector at EOF".
> 
> Fix this by rounding up those partial sectors.
> 
> There are two possible alternative fixes:
> (1) We could refuse to open unaligned image files with O_DIRECT
>     altogether.  That sounds reasonable until you realize that qcow2
>     does necessarily not fill up its metadata clusters, and that nobody
>     runs qemu-img create with O_DIRECT.  Therefore, unpreallocated qcow2
>     files usually have an unaligned image tail.

Yep, non-starter.

> 
> (2) bdrv_co_block_status() could ignore unaligned tails.  It actually
>     throws away everything past the EOF already, so that sounds
>     reasonable.
>     Unfortunately, the block layer knows file lengths only with a
>     granularity of BDRV_SECTOR_SIZE, so bdrv_co_block_status() usually
>     would have to guess whether its file length information is inexact
>     or whether the driver is broken.

Well, if I ever get around to my thread of making the block layer honor
byte-accurate sizes, instead of rounding up, then there is no longer
than inexactness. I think our mails crossed, and you missed another idea
of mine of having block drivers (probably only file-posix, per your
audit) set BDRV_BLOCK_EOF when returning an unaligned answer due to EOF,
as the key for letting the block layer know whether the unaligned answer
was due to size rounding.

> 
> Fixing what raw_co_block_status() returns is the safest thing to do.

Agree.

> 
> There seems to be no other block driver that sets request_alignment and
> does not make sure that it always returns aligned values.

Thanks for auditing.

> 
> Cc: address@hidden
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  block/file-posix.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index e09e15bbf8..f489a5420c 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2488,6 +2488,9 @@ static int coroutine_fn 
> raw_co_block_status(BlockDriverState *bs,
>      off_t data = 0, hole = 0;
>      int ret;
>  
> +    assert(QEMU_IS_ALIGNED(offset, bs->bl.request_alignment) &&
> +           QEMU_IS_ALIGNED(bytes, bs->bl.request_alignment));
> +

Can write in one line as:

assert(QEMU_IS_ALIGNED(offset | bytes, bs->bl.request_alignment));

>      ret = fd_open(bs);
>      if (ret < 0) {
>          return ret;
> @@ -2513,6 +2516,20 @@ static int coroutine_fn 
> raw_co_block_status(BlockDriverState *bs,
>          /* On a data extent, compute bytes to the end of the extent,
>           * possibly including a partial sector at EOF. */
>          *pnum = MIN(bytes, hole - offset);
> +
> +        /*
> +         * We are not allowed to return partial sectors, though, so
> +         * round up if necessary.
> +         */
> +        if (!QEMU_IS_ALIGNED(*pnum, bs->bl.request_alignment)) {
> +            int64_t file_length = raw_getlength(bs);
> +            if (file_length > 0) {
> +                /* Ignore errors, this is just a safeguard */
> +                assert(hole == file_length);
> +            }
> +            *pnum = ROUND_UP(*pnum, bs->bl.request_alignment);
> +        }

Reviewed-by: Eric Blake <address@hidden>

bl.request_alignment is normally 1 (making this a no-op), but is
definitely larger for O_DIRECT images (where rounding up and treating
the post-EOF hole the same as the rest of the sector is the same thing
that NBD chose to do).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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