dm cache: emit a warning message if there are a lot of cache blocks

Loading and saving millions of block mappings takes time.  We may as
well explain what's going on, and encourage people to use a larger
cache block size.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 6e36a07531056acd027875c954296e55eb5b4434..abdd45d07bf66e5a217ee041aaa8cdb7e7b78ddf 100644 (file)
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -2301,6 +2301,19 @@ static sector_t calculate_discard_block_size(sector_t cache_block_size,
        return discard_block_size;
 }
 
+static void set_cache_size(struct cache *cache, dm_cblock_t size)
+{
+       dm_block_t nr_blocks = from_cblock(size);
+
+       if (nr_blocks > (1 << 20) && cache->cache_size != size)
+               DMWARN_LIMIT("You have created a cache device with a lot of individual cache blocks (%llu)\n"
+                            "All these mappings can consume a lot of kernel memory, and take some time to read/write.\n"
+                            "Please consider increasing the cache block size to reduce the overall cache block count.",
+                            (unsigned long long) nr_blocks);
+
+       cache->cache_size = size;
+}
+
 #define DEFAULT_MIGRATION_THRESHOLD 2048
 
 static int cache_create(struct cache_args *ca, struct cache **result)
@@ -2356,10 +2369,10 @@ static int cache_create(struct cache_args *ca, struct cache **result)
 
                cache->sectors_per_block_shift = -1;
                cache_size = block_div(cache_size, ca->block_size);
-               cache->cache_size = to_cblock(cache_size);
+               set_cache_size(cache, to_cblock(cache_size));
        } else {
                cache->sectors_per_block_shift = __ffs(ca->block_size);
-               cache->cache_size = to_cblock(ca->cache_sectors >> cache->sectors_per_block_shift);
+               set_cache_size(cache, to_cblock(ca->cache_sectors >> cache->sectors_per_block_shift));
        }
 
        r = create_cache_policy(cache, ca, error);
@@ -2856,7 +2869,7 @@ static int resize_cache_dev(struct cache *cache, dm_cblock_t new_size)
                return r;
        }
 
-       cache->cache_size = new_size;
+       set_cache_size(cache, new_size);
 
        return 0;
 }