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

[Kevin Wolf <kwolf@redhat.com>]

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), flags@entry=(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.

> This is because flash_sync_page(), in m25p80.c, now writes with a len of 
> 0x100, which the page size in flash modules. commit 243e6f69c129 
> ("m25p80: Switch to byte-based block access") removed the alignment on 
> BDRV_SECTOR_SIZE. 
> 
> So I think the safest is to restore the alignment on writes. see below.
> If this is ok, I will send a little serie of fixes for m25p80 with this 
> one included.

No, bdrv_co_pwritev() is supposed to handle sub-sector writes correctly,
so this is what we need to fix.

Kevin