On Tue, Oct 20 2009 at 6:46pm -0400,
Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> Simplify exception handover so that the duplicate snapshot is found in
> one location (snapshot_ctr) and recorded in the associated (old and new)
> snapshots that will be participating in the handover.
>
> Confines handover to be from a specific snapshot to another specific
> snapshot without additional __find_duplicate() calls.
>
> Documents the exception handover state diagram with relevant comments
> in the associated code.
In testing --onactivate support I found that my revised handover
incorrectly assumed that the snapshot-merge target _always_ needs the
exceptions explicitly handed over to it.
In reality --onactivate doesn't require any handover at all. The
snapshot_ctr() for such a merge is provided with the correct cow_path up
front; so no handover is needed.
I also found that I was missing some locking when changing the "old"
snapshot's 'handover_snap' reference.
The following incremental patch addresses both these oversights; I'll be
posting v3 of this "dm snapshot: exception handover improvements"
shortly (it will include these fixes).
diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index fca2479..e0330e3 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -982,13 +982,13 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
}
/* cross reference snapshots that will do handover */
+ down_write(&dup->lock);
dup->handover_snap = s;
+ up_write(&dup->lock);
s->handover_snap = dup;
+
/* this new snapshot will accept the handover */
s->handover = 1;
- } else if (is_merge(ti)) {
- ti->error = "Unable to find snapshot that is to be merged";
- goto bad_load_and_register;
}
/* Metadata must only be loaded into one table at once */
--
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