[dm-devel] dm-cache writethrough issue.

[Joe Thornber]

Alasdair,

Could you push this upstream ASAP please?

I'm not happy that we're now adding such a large structure to the per
bio data, at the very least we should only do this if writethrough is
enabled.  But I'll improve this later, for now correctness is the
important thing.

- Joe



Author: Joe Thornber <ejt at redhat.com>
Date:   Mon Mar 25 10:10:58 2013 +0000

    [dm-cache] Writethrough mode wasn't resetting bio fields before reusing
    
    The writethrough mode reuses the same bio to write to both the origin
    and cache device.  It was recording the bi_sector and restoring this,
    but this was inadequate; lower level drivers can change all sorts of
    bio fields.
    
    This patch adds a struct dm_bio_details field, and uses
    dm_bio_record() and dm_bio_restore() to ensure the bio is restored
    before reissuing to the cache device.
    
    Issue discovered by Darrick Wong.

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index cf24a46..873f495 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -6,6 +6,7 @@
 
 #include "dm.h"
 #include "dm-bio-prison.h"
+#include "dm-bio-record.h"
 #include "dm-cache-metadata.h"
 
 #include <linux/dm-io.h>
@@ -205,6 +206,7 @@ struct per_bio_data {
        struct cache *cache;
        dm_cblock_t cblock;
        bio_end_io_t *saved_bi_end_io;
+       struct dm_bio_details bio_details;
 };
 
 struct dm_cache_migration {
@@ -642,6 +644,7 @@ static void writethrough_endio(struct bio *bio, int err)
                return;
        }
 
+       dm_bio_restore(&pb->bio_details, bio);
        remap_to_cache(pb->cache, bio, pb->cblock);
 
        /*
@@ -665,6 +668,7 @@ static void remap_to_origin_then_cache(struct cache *cache, struct bio *bio,
        pb->cache = cache;
        pb->cblock = cblock;
        pb->saved_bi_end_io = bio->bi_end_io;
+       dm_bio_record(&pb->bio_details, bio);
        bio->bi_end_io = writethrough_endio;
 
        remap_to_origin_clear_discard(pb->cache, bio, oblock);