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Re: [Qemu-block] Unaligned images with O_DIRECT

From: Max Reitz
Subject: Re: [Qemu-block] Unaligned images with O_DIRECT
Date: Tue, 14 May 2019 19:28:16 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

On 14.05.19 18:15, Max Reitz wrote:
> On 14.05.19 17:45, Eric Blake wrote:
>> On 5/14/19 10:06 AM, Max Reitz wrote:
>>> Hi,
>>> Unaligned images don’t work so well with O_DIRECT:
>>> $ 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.
>>> [1]    10954 abort (core dumped)  qemu-img map --image-opts
>>> driver=file,filename=foo,cache.direct=on
>>> (compare https://bugzilla.redhat.com/show_bug.cgi?id=1588356)
>>> This is because the request_alignment is 512 (in my case), but the EOF
>>> is not aligned accordingly, so raw_co_block_status() returns an aligned
>>> *pnum.
>> Uggh. Yet another reason why I want qemu to support byte-accurate
>> sizing, instead of rounding up. The rounding keeps raising its head in
>> more and more places. I have pending patches that are trying to improve
>> block status to round driver answers up to match request_alignment (when
>> the protocol layer has finer granularity than the format layer); but
>> this sounds like it is a bug in the file driver itself for returning an
>> answer that is not properly rounded according to its own
>> request_alignment boundary, and not one where my pending patches would help.
> Yes, I think so, too.
>>> I suppose having an unaligned tail is not so bad and maybe we can just
>>> adjust the assertion accordingly.  On the other hand, this has been
>>> broken for a while.  Does it even make sense to use O_DIRECT with
>>> unaligned images?  Shouldn’t we just reject them outright?
>> The tail of an unaligned file is generally inaccessible to O_DIRECT,
> Especially with this.
>> where it is easier to use ftruncate() up to an aligned boundary if you
>> really must play with that region of the file, and then ftruncate() back
>> to the intended size after I/O. But that sounds hairy.  We could also
>> round down and silently ignore the tail of the file, but that is at odds
>> with our practice of rounding size up.  So for the short term, I'd be
>> happy with a patch that just rejects any attempt to use cache.direct=on
>> (O_DIRECT) with a file that is not already a multiple of the alignment
>> required thereby. (For reference, that's what qemu as NBD client
>> recently did when talking to a server that advertises a size
>> inconsistent with forced minimum block access: commit 3add3ab7)
> OK, I’ll send a patch.  Thanks for you explanation!
Well, or maybe not.

$ ./qemu-img create -f qcow2 foo.qcow2 64M
$ ./qemu-img map --image-opts \
qemu-img: Could not open
'driver=qcow2,file.filename=foo.qcow2,cache.direct=on': File length
(196616 bytes) is not a multiple of the O_DIRECT alignment (512 bytes)
Try cache.direct=off, or increasing the file size to match the alignment

That may be considered a bug in qcow2.  Maybe it should always fill all
clusters.  But even if we did so and fixed it now, we can’t disallow
qemu from opening such images.

Also, well, the tail is accessible, we just need to access it with the
proper alignment (and then we get a short read).  This seems to be
handled just fine.

So I think file-posix should just return a rounded result.  Well, or
bdrv_co_Block_status() could ignore it for the EOF, because it throws
away everything past the EOF anyway with:

    if (*pnum > bytes) {
        *pnum = bytes;

On one hand, I agree that file-posix should return an aligned result.
On the other, it doesn’t make a difference, so I don’t think we need to
enforce it (at EOF).


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