mbcache: Limit the maximum number of cache entries

Limit the maximum number of mb_cache entries depending on the number of
hash buckets: if the only limit to the number of cache entries is the
available memory the hash chains can grow very long, taking a long time
to search.

At least partially solves https://bugzilla.lustre.org/show_bug.cgi?id=22771.

Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 24 insertions, 6 deletions

diff --git a/fs/mbcache.c b/fs/mbcache.c
index cf4e6cdfd15b..93444747237b 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -80,6 +80,7 @@ struct mb_cache {
         struct list_head                c_cache_list;
         const char                      *c_name;
         atomic_t                        c_entry_count;
+        int                             c_max_entries;
         int                             c_bucket_bits;
         struct kmem_cache               *c_entry_cache;
         struct list_head                *c_block_hash;
@@ -243,6 +244,12 @@ mb_cache_create(const char *name, int bucket_bits)
         if (!cache->c_entry_cache)
                 goto fail2;
 
+        /*
+         * Set an upper limit on the number of cache entries so that the hash
+         * chains won't grow too long.
+         */
+        cache->c_max_entries = bucket_count << 4;
+
         spin_lock(&mb_cache_spinlock);
         list_add(&cache->c_cache_list, &mb_cache_list);
         spin_unlock(&mb_cache_spinlock);
@@ -333,7 +340,6 @@ mb_cache_destroy(struct mb_cache *cache)
         kfree(cache);
 }
 
-
 /*
  * mb_cache_entry_alloc()
  *
@@ -345,17 +351,29 @@ mb_cache_destroy(struct mb_cache *cache)
 struct mb_cache_entry *
 mb_cache_entry_alloc(struct mb_cache *cache, gfp_t gfp_flags)
 {
-        struct mb_cache_entry *ce;
-
-        ce = kmem_cache_alloc(cache->c_entry_cache, gfp_flags);
-        if (ce) {
+        struct mb_cache_entry *ce = NULL;
+
+        if (atomic_read(&cache->c_entry_count) >= cache->c_max_entries) {
+                spin_lock(&mb_cache_spinlock);
+                if (!list_empty(&mb_cache_lru_list)) {
+                        ce = list_entry(mb_cache_lru_list.next,
+                                        struct mb_cache_entry, e_lru_list);
+                        list_del_init(&ce->e_lru_list);
+                        __mb_cache_entry_unhash(ce);
+                }
+                spin_unlock(&mb_cache_spinlock);
+        }
+        if (!ce) {
+                ce = kmem_cache_alloc(cache->c_entry_cache, gfp_flags);
+                if (!ce)
+                        return NULL;
                 atomic_inc(&cache->c_entry_count);
                 INIT_LIST_HEAD(&ce->e_lru_list);
                 INIT_LIST_HEAD(&ce->e_block_list);
                 ce->e_cache = cache;
-                ce->e_used = 1 + MB_CACHE_WRITER;
                 ce->e_queued = 0;
         }
+        ce->e_used = 1 + MB_CACHE_WRITER;
         return ce;
 }