Re: [RFC][PATCH] md: avoid fullsync if a faulty member missed a dirty transition

[Neil Brown <neilb@suse.de>]

On Tuesday May 6, snitzer@gmail.com wrote:
> 
> It looks like bitmap_update_sb()'s incrementing of events_cleared (on
> behalf of the local member) could be racing with the fact that the NBD
> member becomes faulty (whereby making the array degraded).  This
> allows the events_cleared to reflect a clean->dirty transition last
> occurred before the array became degraded.  My reasoning is: If it was
> a clean->dirty transition the bitmap still has the associated dirty
> bit set in the local member's bitmap, so using the bitmap to resync is
> valid.
> 
> thanks,
> Mike

Thanks for persisting.  I think I understand what is going on now.

How about this patch?  It is similar to your, but instead of depending
on the odd/even state of the event counter, it directly checks the
clean/dirty state of the array.

NeilBrown


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

### Diffstat output
 ./drivers/md/md.c |    5 +++++
 1 file changed, 5 insertions(+)

diff .prev/drivers/md/md.c ./drivers/md/md.c
--- .prev/drivers/md/md.c	2008-05-02 14:49:05.000000000 +1000
+++ ./drivers/md/md.c	2008-05-08 16:10:48.000000000 +1000
@@ -843,6 +843,8 @@ static int super_90_validate(mddev_t *md
 		/* if adding to array with a bitmap, then we can accept an
 		 * older device ... but not too old.
 		 */
+		if (sb->state & (1<<MD_SB_CLEAN))
+			ev1++;
 		if (ev1 < mddev->bitmap->events_cleared)
 			return 0;
 	} else {
@@ -1218,6 +1220,9 @@ static int super_1_validate(mddev_t *mdd
 		/* If adding to array with a bitmap, then we can accept an
 		 * older device, but not too old.
 		 */
+		if (mddev->recovery_cp == MaxSector)
+			/* array was clean, so can allow 'next' event */
+			ev1++;
 		if (ev1 < mddev->bitmap->events_cleared)
 			return 0;
 	} else {