Re: [Qemu-devel] [PATCH] m25p80: fix test on blk_pread() return value

[Cédric Le Goater]

On 06/14/2016 10:38 AM, Kevin Wolf wrote:
> Am 14.06.2016 um 10:02 hat Cédric Le Goater geschrieben:
>>>> #4  0x00007fa81c6694ac in bdrv_aligned_pwritev (bs=0x7fa81d4dd050, 
>>>> req=<optimized out>, offset=30878208, 
>>>>     bytes=512, qiov=0x7fa7f47fee60, flags=0)
>>>>     at /home/legoater/work/qemu/qemu-ast2400-mainline.git/block/io.c:1243
>>>> #5  0x00007fa81c669ecb in bdrv_co_pwritev (bs=0x7fa81d4dd050, offset=8, 
>>>> bytes=512, qiov=0x7fa80d5191c0, 
>>>>     flags=(BDRV_REQ_COPY_ON_READ | BDRV_REQ_ZERO_WRITE | 
>>>> BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_SERIALISING | BDRV_REQ_FUA | unknown: 
>>>> 4278124256), address@hidden(unknown: 0))
>>>>     at /home/legoater/work/qemu/qemu-ast2400-mainline.git/block/io.c:1492
>>>
>>> That 'flags' value looks bogus...
>>>
>>>> #6  0x00007fa81c65e367 in blk_co_pwritev (blk=0x7fa81d4c5b60, 
>>>> offset=30878208, bytes=256, qiov=0x7fa80d5191c0, 
>>>>     flags=(unknown: 0)) at 
>>>> /home/legoater/work/qemu/qemu-ast2400-mainline.git/block/block-backend.c:788
>>>> #7  0x00007fa81c65e49b in blk_aio_write_entry (opaque=0x7fa7e849aca0)
>>>>     at 
>>>> /home/legoater/work/qemu/qemu-ast2400-mainline.git/block/block-backend.c:977
>>>> #8  0x00007fa81c6c823a in coroutine_trampoline (i0=<optimized out>, 
>>>> i1=<optimized out>)
>>>>     at 
>>>> /home/legoater/work/qemu/qemu-ast2400-mainline.git/util/coroutine-ucontext.c:78
>>>> #9  0x00007fa818ea8f00 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
>>>
>>> and we don't get anything further in the backtrace beyond coroutines, to
>>> see who's sending the bad parameters.  I recently debugged a bogus flags
>>> in bdrv_aio_preadv, by hoisting an assert to occur before coroutines are
>>> used in blk_aio_prwv():
>>>
>>> https://lists.gnu.org/archive/html/qemu-devel/2016-06/msg02948.html
>>>
>>> I've just posted v2 of that patch (now a 2/2 series), but in v2 no
>>> longer kept the assert at that point.  But maybe the correct fix, and/or
>>> the hack for catching the bug prior to coroutines, will help you debug
>>> where the bad arguments are coming from.
>>
>> That does not fix the assert.
>>  
>>>> #10 0x00007fa80d5189d0 in ?? ()
>>>> #11 0x0000000000000000 in ?? ()
>>>> (gdb) up 4
>>>> #4  0x00007fa81c6694ac in bdrv_aligned_pwritev (bs=0x7fa81d4dd050, 
>>>> req=<optimized out>, offset=30878208, 
>>>>     bytes=512, qiov=0x7fa7f47fee60, flags=0)
>>>>     at /home/legoater/work/qemu/qemu-ast2400-mainline.git/block/io.c:1243
>>>> 1243           assert(!qiov || bytes == qiov->size);
>>>> (gdb) p *qiov 
>>>> $1 = {iov = 0x7fa81da671d0, niov = 1, nalloc = 1, size = 256}
>>
>> So, it seems that the issue is coming from the fact that bdrv_co_pwritev()
>> does not handle alignments less than BDRV_SECTOR_SIZE :
>>
>>      /* TODO Lift BDRV_SECTOR_SIZE restriction in BlockDriver interface */
>>      uint64_t align = MAX(BDRV_SECTOR_SIZE, bs->request_alignment);
>>
>> It calls bdrv_aligned_pwritev() which does the assert : 
>>
>>      assert(!qiov || bytes == qiov->size);
> 
> Yes, but between these two places, there is code that should actually
> enforce the right alignment:
> 
>     if ((offset + bytes) & (align - 1)) {
>         ...
>     }
> 
> You can see in your backtrace that bdrv_aligned_pwritev() gets a
> different qiov than bdrv_co_pwritev() (which is local_qiov in the latter
> function).
> 
> It's just unclear to me why this code extended bytes, but didn't add the
> tail_buf iovec to local_qiov.

The gdb backtrace is bogus. It does not make sense. May be a gdb issue
with multithread on jessie.

In the path tracking the tail bytes, we have : 

     if ((offset + bytes) & (align - 1)) {
        ...
        tail_bytes = (offset + bytes) & (align - 1);
        qemu_iovec_add(&local_qiov, tail_buf + tail_bytes, align - tail_bytes);

        bytes = ROUND_UP(bytes, align);
    }

This is where the issue is I think. The qiov holds 256 and bytes 512.

I have no idea how to fix that though.

Thanks,

C.