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