dm cache: fix resize crash if user doesn't reload cache table

This change “dm cache: fix resize crash if user doesn’t reload cache table” in Linux kernel is authored by Mike Snitzer <snitzer [at]> on Tue Sep 25 20:56:02 2018 -0400.

dm cache: fix resize crash if user doesn't reload cache table

A reload of the cache's DM table is needed during resize because
otherwise a crash will occur when attempting to access smq policy
entries associated with the portion of the cache that was recently

The reason is cache-size based data structures in the policy will not be
resized, the only way to safely extend the cache is to allow for a
proper cache policy initialization that occurs when the cache table is
loaded.  For example the smq policy's space_init(), init_allocator(),
calc_hotspot_params() must be sized based on the extended cache size.

The fix for this is to disallow cache resizes of this pattern:
1) suspend "cache" target's device
2) resize the fast device used for the cache
3) resume "cache" target's device

Instead, the last step must be a full reload of the cache's DM table.

Fixes: 66a636356 ("dm cache: add stochastic-multi-queue (smq) policy")
Signed-off-by: Mike Snitzer <>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 5d07384.

There are 9 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/md/dm-cache-target.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index a534133..e13d991 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -3009,8 +3009,13 @@ static dm_cblock_t get_cache_dev_size(struct cache *cache)
 static bool can_resize(struct cache *cache, dm_cblock_t new_size)
-	if (from_cblock(new_size) > from_cblock(cache->cache_size))
-		return true;
+	if (from_cblock(new_size) > from_cblock(cache->cache_size)) {
+		if (cache->sized) {
+			DMERR("%s: unable to extend cache due to missing cache table reload",
+			      cache_device_name(cache));
+			return false;
+		}
+	}
 	 * We can't drop a dirty block when shrinking the cache.

The commit for this change in Linux stable tree is 5d07384 (patch).

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