Am 14.06.2016 um 18:02 hat Cédric Le Goater geschrieben: > 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), 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. > > 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)) { > ... if (!use_local_qiov) { qemu_iovec_init(&local_qiov, qiov->niov + 1); qemu_iovec_concat(&local_qiov, qiov, 0, qiov->size); use_local_qiov = true; } > 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.
Added some more context above. qiov->size as passed from the device is already 256 bytes, which are added to local_qiov with qemu_iovec_concat(). And then we add another 256 from tail_buf in the lines that you quoted, so in theory we should end up with a properly aligned 256 + 256 = 512 byte qiov. Kevin