On Sun, 21 Feb 2010 19:16:40 +1100
Neil Brown <neilb@xxxxxxx> wrote:
> On Sun, 21 Feb 2010 02:26:42 -0500
> chris <tknchris@xxxxxxxxx> wrote:
>
> > That is exactly what I didn't want to hear :( I am running
> > 2.6.26-2-xen-amd64. Are you sure its a kernel problem and nothing to
> > do with my chunk/block sizes? If this is a bug what versions are
> > affected, I'll build a new domU kernel and see if I can get it working
> > there.
> >
> > - chris
>
> I'm absolutely sure it is a kernel bug.
And I think I now know what the bug is.
A patch was recently posted to dm-devel which I think addresses exactly this
problem.
I reproduce it below.
NeilBrown
-------------------
If the lower device exposes a merge_bvec_fn,
dm_set_device_limits() restricts max_sectors
to PAGE_SIZE "just to be safe".
This is not sufficient, however.
If someone uses bio_add_page() to add 8 disjunct 512 byte partial
pages to a bio, it would succeed, but could still cross a border
of whatever restrictions are below us (e.g. raid10 stripe boundary).
An attempted bio_split() would not succeed, because bi_vcnt is 8.
One example that triggered this frequently is the xen io layer.
raid10_make_request bug: can't convert block across chunks or bigger than 64k 209265151 1
Signed-off-by: Lars <lars.ellenberg@xxxxxxxxxx>
---
drivers/md/dm-table.c | 12 ++++++++++--
1 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 4b22feb..c686ff4 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -515,14 +515,22 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
/*
* Check if merge fn is supported.
- * If not we'll force DM to use PAGE_SIZE or
+ * If not we'll force DM to use single bio_vec of PAGE_SIZE or
* smaller I/O, just to be safe.
*/
- if (q->merge_bvec_fn && !ti->type->merge)
+ if (q->merge_bvec_fn && !ti->type->merge) {
limits->max_sectors =
min_not_zero(limits->max_sectors,
(unsigned int) (PAGE_SIZE >> 9));
+ /* Restricting max_sectors is not enough.
+ * If someone uses bio_add_page to add 8 disjunct 512 byte
+ * partial pages to a bio, it would succeed,
+ * but could still cross a border of whatever restrictions
+ * are below us (e.g. raid0 stripe boundary). An attempted
+ * bio_split() would not succeed, because bi_vcnt is 8. */
+ limits->max_segments = 1;
+ }
return 0;
}
EXPORT_SYMBOL_GPL(dm_set_device_limits);
--
1.6.3.3
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