Re: [Qemu-devel] [Bug 1343827] [NEW] block.c: multiwrite_merge() truncates overlapping requests

[Andrey Korolyov]

On Fri, Jul 18, 2014 at 10:15 AM, Slava Pestov
<address@hidden> wrote:
> Public bug reported:
>
> If the list of requests passed to multiwrite_merge() contains two
> requests where the first is for a range of sectors that is a strict
> subset of the second's, the second request is truncated to end where the
> first starts, so the second half of the second request is lost.
>
> This is easy to reproduce by running fio against a virtio-blk device
> running on qemu 2.1.0-rc1 with the below fio script. At least with fio
> 2.0.13, the randwrite pass will issue overlapping bios to the block
> driver, which the kernel is happy to pass along to qemu:
>
> [global]
> randrepeat=0
> ioengine=libaio
> iodepth=64
> direct=1
> size=1M
> numjobs=1
> verify_fatal=1
> verify_dump=1
>
> filename=$dev
>
> [seqwrite]
> blocksize_range=4k-1M
> rw=write
> verify=crc32c-intel
>
> [randwrite]
> stonewall
> blocksize_range=4k-1M
> rw=randwrite
> verify=meta
>
> Here is a naive fix for the problem that simply avoids merging
> problematic requests. I guess a better solution would be to redo
> qemu_iovec_concat() to do the right thing.
>
> diff -ur old/qemu-2.1.0-rc2/block.c qemu-2.1.0-rc2/block.c
> --- old/qemu-2.1.0-rc2/block.c  2014-07-15 14:49:14.000000000 -0700
> +++ qemu-2.1.0-rc2/block.c      2014-07-17 23:03:14.224169741 -0700
> @@ -4460,7 +4460,9 @@
>          int64_t oldreq_last = reqs[outidx].sector + reqs[outidx].nb_sectors;
>
>          // Handle exactly sequential writes and overlapping writes.
> -        if (reqs[i].sector <= oldreq_last) {
> +        // If this request ends before the previous one, don't merge.
> +        if (reqs[i].sector <= oldreq_last &&
> +            reqs[i].sector + reqs[i].nb_sectors >= oldreq_last) {
>              merge = 1;
>          }
>
> ** Affects: qemu
>      Importance: Undecided
>          Status: New
>
> --
> You received this bug notification because you are a member of qemu-
> devel-ml, which is subscribed to QEMU.
> https://bugs.launchpad.net/bugs/1343827
>
> Title:
>   block.c: multiwrite_merge() truncates overlapping requests
>
> Status in QEMU:
>   New
>
> Bug description:
>   If the list of requests passed to multiwrite_merge() contains two
>   requests where the first is for a range of sectors that is a strict
>   subset of the second's, the second request is truncated to end where
>   the first starts, so the second half of the second request is lost.
>
>   This is easy to reproduce by running fio against a virtio-blk device
>   running on qemu 2.1.0-rc1 with the below fio script. At least with fio
>   2.0.13, the randwrite pass will issue overlapping bios to the block
>   driver, which the kernel is happy to pass along to qemu:
>
>   [global]
>   randrepeat=0
>   ioengine=libaio
>   iodepth=64
>   direct=1
>   size=1M
>   numjobs=1
>   verify_fatal=1
>   verify_dump=1
>
>   filename=$dev
>
>   [seqwrite]
>   blocksize_range=4k-1M
>   rw=write
>   verify=crc32c-intel
>
>   [randwrite]
>   stonewall
>   blocksize_range=4k-1M
>   rw=randwrite
>   verify=meta
>
>   Here is a naive fix for the problem that simply avoids merging
>   problematic requests. I guess a better solution would be to redo
>   qemu_iovec_concat() to do the right thing.
>
>   diff -ur old/qemu-2.1.0-rc2/block.c qemu-2.1.0-rc2/block.c
>   --- old/qemu-2.1.0-rc2/block.c  2014-07-15 14:49:14.000000000 -0700
>   +++ qemu-2.1.0-rc2/block.c      2014-07-17 23:03:14.224169741 -0700
>   @@ -4460,7 +4460,9 @@
>            int64_t oldreq_last = reqs[outidx].sector + 
> reqs[outidx].nb_sectors;
>
>            // Handle exactly sequential writes and overlapping writes.
>   -        if (reqs[i].sector <= oldreq_last) {
>   +        // If this request ends before the previous one, don't merge.
>   +        if (reqs[i].sector <= oldreq_last &&
>   +            reqs[i].sector + reqs[i].nb_sectors >= oldreq_last) {
>                merge = 1;
>            }
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/qemu/+bug/1343827/+subscriptions
>

Hello,

bug is still here in the master and can be easily reproduced (and, of
course, looks like blocker since data corruption takes place). Does
anyone have an idea on when the fix (at least suggested one) will be
merged?