[RFC PATCH v3 2/4] mm/zswap: use zsmalloc deferred free callback for async invalidate

[Wenchao Hao <haowenchao22@gmail.com>]

Register zswap_deferred_ops to defer the entire zswap_entry_free()
to the WQ_UNBOUND worker. The invalidate hot path only stores the
entry pointer into the per-cpu buffer (512 entries/page).

The drain callback performs the full entry teardown: lru_del, zs_free,
memcg uncharge, cache_free, and stats update. On deferred failure,
fallback to synchronous zswap_entry_free().

Signed-off-by: Wenchao Hao <haowenchao@xiaomi.com>
---
 mm/zswap.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 4b5149173b0e..3f23ddbe525c 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -270,6 +270,8 @@ static void acomp_ctx_free(struct crypto_acomp_ctx *acomp_ctx)
 	acomp_ctx->buffer = NULL;
 }
 
+static const struct zs_deferred_ops zswap_deferred_ops;
+
 static struct zswap_pool *zswap_pool_create(char *compressor)
 {
 	struct zswap_pool *pool;
@@ -289,6 +291,8 @@ static struct zswap_pool *zswap_pool_create(char *compressor)
 	if (!pool->zs_pool)
 		goto error;
 
+	zs_pool_enable_deferred_free(pool->zs_pool, &zswap_deferred_ops, pool);
+
 	strscpy(pool->tfm_name, compressor, sizeof(pool->tfm_name));
 
 	/* Many things rely on the zero-initialization. */
@@ -777,6 +781,36 @@ static void zswap_entry_free(struct zswap_entry *entry)
 	atomic_long_dec(&zswap_stored_pages);
 }
 
+static enum zs_push_ret zswap_deferred_push(void *buf,
+		unsigned int count, unsigned long value)
+{
+	unsigned long *entries = buf;
+
+	if (count >= PAGE_SIZE / sizeof(unsigned long))
+		return ZS_PUSH_FULL;
+	entries[count] = value;
+	if (count + 1 >= PAGE_SIZE / sizeof(unsigned long))
+		return ZS_PUSH_FULL_QUEUED;
+	return ZS_PUSH_OK;
+}
+
+static void zswap_deferred_drain(void *private, void *buf, unsigned int count)
+{
+	unsigned long *entries = buf;
+	unsigned int i;
+
+	for (i = 0; i < count; i++) {
+		struct zswap_entry *entry = (struct zswap_entry *)entries[i];
+
+		zswap_entry_free(entry);
+	}
+}
+
+static const struct zs_deferred_ops zswap_deferred_ops = {
+	.push = zswap_deferred_push,
+	.drain = zswap_deferred_drain,
+};
+
 /*********************************
 * compressed storage functions
 **********************************/
@@ -1647,7 +1681,9 @@ void zswap_invalidate(swp_entry_t swp)
 		return;
 
 	entry = xa_erase(tree, offset);
-	if (entry)
+	if (!entry)
+		return;
+	if (!zs_free_deferred(entry->pool->zs_pool, (unsigned long)entry))
 		zswap_entry_free(entry);
 }
 
-- 
2.34.1