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SubjectRe: [RFC][PATCH] md: avoid fullsync if a faulty member missed a dirty transition
On Thursday May 8, snitzer@gmail.com wrote:
> On Thu, May 8, 2008 at 2:13 AM, Neil Brown <neilb@suse.de> wrote:
> > 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.
>
> Hi Neil,
>
> Your revised patch works great and is obviously cleaner.

But I'm still not happy with it :-(
I suspect there might be other cases where it will still do the wrong
thing.
The real problem is that we are updating events_cleared to early. We
are setting to the new event counter before that is even written out.

So I've come up with this patch, which I think more clearly
encapsulated what events_cleared means. It is now set to the current
'events' counter immediately before we clear any bit.

If you could test it, I'd really appreciate it.

Thanks,
NeilBrown

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

### Diffstat output
./drivers/md/bitmap.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)
diff .prev/drivers/md/bitmap.c ./drivers/md/bitmap.c
--- .prev/drivers/md/bitmap.c 2008-05-09 11:02:13.000000000 +1000
+++ ./drivers/md/bitmap.c 2008-05-09 11:38:35.000000000 +1000
@@ -465,8 +465,6 @@ void bitmap_update_sb(struct bitmap *bit
spin_unlock_irqrestore(&bitmap->lock, flags);
sb = (bitmap_super_t *)kmap_atomic(bitmap->sb_page, KM_USER0);
sb->events = cpu_to_le64(bitmap->mddev->events);
- if (!bitmap->mddev->degraded)
- sb->events_cleared = cpu_to_le64(bitmap->mddev->events);
kunmap_atomic(sb, KM_USER0);
write_page(bitmap, bitmap->sb_page, 1);
}
@@ -1094,9 +1092,19 @@ void bitmap_daemon_work(struct bitmap *b
} else
spin_unlock_irqrestore(&bitmap->lock, flags);
lastpage = page;
-/*
- printk("bitmap clean at page %lu\n", j);
-*/
+
+ /* We are possibly going to clear some bits, so make
+ * sure that events_cleared is up-to-date.
+ */
+ if (bitmap->events_cleared < bitmap->mddev->events) {
+ bitmap_super_t *sb;
+ sb = kmap_atomic(bitmap->sb_page, KM_USER0);
+ bitmap->events_cleared = bitmap->mddev->events;
+ sb->events_cleared =
+ cpu_to_le64(bitmap->events_cleared);
+ kunmap_atomic(sb, KM_USER0);
+ write_page(bitmap, bitmap->sb_page, 1);
+ }
spin_lock_irqsave(&bitmap->lock, flags);
clear_page_attr(bitmap, page, BITMAP_PAGE_CLEAN);
}

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