linux-ext4 - [PATCH 15/20] cache: support gradual expansion

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Message-ID: <175573713077.20753.2190936945232727449.stgit@frogsfrogsfrogs>
Date: Wed, 20 Aug 2025 18:11:47 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: John@...ves.net, bernd@...ernd.com, linux-fsdevel@...r.kernel.org,
 linux-ext4@...r.kernel.org, miklos@...redi.hu, amir73il@...il.com,
 joannelkoong@...il.com, neal@...pa.dev
Subject: [PATCH 15/20] cache: support gradual expansion

From: Darrick J. Wong <djwong@...nel.org>

It's probably not a good idea to expand the cache size by powers of two
beyond some random limit, so let the users figure that out if they want
to.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 lib/support/cache.h |   10 ++++++++++
 lib/support/cache.c |   12 ++++++++++--
 2 files changed, 20 insertions(+), 2 deletions(-)


diff --git a/lib/support/cache.h b/lib/support/cache.h
index 98b2182d49a6e0..ae37945c545f46 100644
--- a/lib/support/cache.h
+++ b/lib/support/cache.h
@@ -66,6 +66,14 @@ typedef int (*cache_node_compare_t)(struct cache_node *, cache_key_t);
 typedef unsigned int (*cache_bulk_relse_t)(struct cache *, struct list_head *);
 typedef int (*cache_node_get_t)(struct cache *c, struct cache_node *cn);
 typedef void (*cache_node_put_t)(struct cache *c, struct cache_node *cn);
+typedef unsigned int (*cache_node_resize_t)(const struct cache *c,
+					    unsigned int curr_size);
+
+static inline unsigned int cache_gradual_resize(const struct cache *cache,
+						unsigned int curr_size)
+{
+	return curr_size * 5 / 4;
+}
 
 struct cache_operations {
 	cache_node_hash_t	hash;
@@ -76,6 +84,7 @@ struct cache_operations {
 	cache_bulk_relse_t	bulkrelse;	/* optional */
 	cache_node_get_t	get;		/* optional */
 	cache_node_put_t	put;		/* optional */
+	cache_node_resize_t	resize;		/* optional */
 };
 
 struct cache_hash {
@@ -113,6 +122,7 @@ struct cache {
 	cache_bulk_relse_t	bulkrelse;	/* bulk release routine */
 	cache_node_get_t	get;		/* prepare cache node after get */
 	cache_node_put_t	put;		/* prepare to put cache node */
+	cache_node_resize_t	resize;		/* compute new maxcount */
 	unsigned int		c_hashsize;	/* hash bucket count */
 	unsigned int		c_hashshift;	/* hash key shift */
 	struct cache_hash	*c_hash;	/* hash table buckets */
diff --git a/lib/support/cache.c b/lib/support/cache.c
index 9da6c59b3b6391..dbaddc1bd36d3d 100644
--- a/lib/support/cache.c
+++ b/lib/support/cache.c
@@ -62,6 +62,7 @@ cache_init(
 		cache_operations->bulkrelse : cache_generic_bulkrelse;
 	cache->get = cache_operations->get;
 	cache->put = cache_operations->put;
+	cache->resize = cache_operations->resize;
 	pthread_mutex_init(&cache->c_mutex, NULL);
 
 	for (i = 0; i <= CACHE_DIRTY_PRIORITY; i++) {
@@ -90,11 +91,18 @@ static void
 cache_expand(
 	struct cache *		cache)
 {
+	unsigned int		new_size = 0;
+
 	pthread_mutex_lock(&cache->c_mutex);
+	if (cache->resize)
+		new_size = cache->resize(cache, cache->c_maxcount);
+	if (new_size <= cache->c_maxcount)
+		new_size = cache->c_maxcount * 2;
 #ifdef CACHE_DEBUG
-	fprintf(stderr, "doubling cache size to %d\n", 2 * cache->c_maxcount);
+	fprintf(stderr, "increasing cache max size from %u to %u\n",
+			cache->c_maxcount, new_size);
 #endif
-	cache->c_maxcount *= 2;
+	cache->c_maxcount = new_size;
 	pthread_mutex_unlock(&cache->c_mutex);
 }