md/raid10: handle merge_bvec_fn in member devices.

Currently we don't honour merge_bvec_fn in member devices so if there
is one, we force all requests to be single-page at most.
This is not ideal.

So enhance the raid10 merge_bvec_fn to check that function in children
as well.

This introduces a small problem.  There is no locking around calls
the ->merge_bvec_fn and subsequent calls to ->make_request.  So a
device added between these could end up getting a request which
violates its merge_bvec_fn.

Currently the best we can do is synchronize_sched().  This will work
providing no preemption happens.  If there is preemption, we just
have to hope that new devices are largely consistent with old devices.

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 8 insertions, 0 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index 39acfe90cc26..1c2063ccf48e 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -128,6 +128,10 @@ struct md_rdev {
 enum flag_bits {
         Faulty,                 /* device is known to have a fault */
         In_sync,                /* device is in_sync with rest of array */
+        Unmerged,               /* device is being added to array and should
+                                 * be considerred for bvec_merge_fn but not
+                                 * yet for actual IO
+                                 */
         WriteMostly,            /* Avoid reading if at all possible */
         AutoDetected,           /* added by auto-detect */
         Blocked,                /* An error occurred but has not yet
@@ -345,6 +349,10 @@ struct mddev {
         int                             degraded;       /* whether md should consider
                                                          * adding a spare
                                                          */
+        int                             merge_check_needed; /* at least one
+                                                             * member device
+                                                             * has a
+                                                             * merge_bvec_fn */
 
         atomic_t                        recovery_active; /* blocks scheduled, but not written */
         wait_queue_head_t               recovery_wait;