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

[Max Reitz]

On 14.05.19 23:50, Eric Blake wrote:
> 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.

Yes, that EOF change makes sense, I think.  Not least because right now
the EOF detection in block/io.c has to be a bit wonky considering that
it's inexact...  But to be honest, returning the EOF flag from the
drivers would have required me to modify all drivers.  I felt like maybe
that something to be left for another time. :-)

OTOH, I don’t know whether returning the EOF flag from the drivers would
still sense if we had a byte-accurate bdrv_getlength()...

>> 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));

Ah, yeah, sure, why not.

>>      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>

Thanks!

I'll send a v2 with shorter assert().

Max

> 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).

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