On Wed, Mar 03, 2010 at 09:07:34PM +0100, Jens Axboe wrote:
> > 2) What statement "bio_add_page() must accept at least one page"
> > exactly means?
> > IMHO this means that bio_add_page() must accept at least
> > one page with len (PAGE_SIZE - offset). Or more restricted
> > statemnt that first bio_add_page() must be always successfull.
>
> It's really 'first add must succeed', the restriction being that you
> cannot rely on that first add being more than a single page. So the rule
> is that you must accept at least a page at any offset if the bio is
> currently empty, since we know that a page is typically our IO
> granularity.
Speaking of...
dm_set_device_limits is still doing things wrong here, I think.
I posted this about two years ago, but somehow it got lost
and I lost it from my focus as well.
Reading this post reminded me ... there was something:
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 4b22feb..bc34901 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -519,10 +519,22 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
* 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 (raid0 stripe boundary). An attempted
+ * bio_split would not succeed, because bi_vcnt is 8.
+ * E.g. the xen io layer is known to trigger this.
+ */
+ limits->max_segments = 1;
+ }
+
return 0;
}
EXPORT_SYMBOL_GPL(dm_set_device_limits);
Thanks,
Lars
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