[PATCH 6.12.y 2/2] mm: zswap: move allocations during CPU init outside the lock

[Yosry Ahmed <yosryahmed@google.com>]

In zswap_cpu_comp_prepare(), allocations are made and assigned to various
members of acomp_ctx under acomp_ctx->mutex.  However, allocations may
recurse into zswap through reclaim, trying to acquire the same mutex and
deadlocking.

Move the allocations before the mutex critical section.  Only the
initialization of acomp_ctx needs to be done with the mutex held.

Link: https://lkml.kernel.org/r/20250113214458.2123410-1-yosryahmed@google.com
Fixes: 12dcb0ef5406 ("mm: zswap: properly synchronize freeing resources during CPU hotunplug")
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 779b9955f64327c339a16f68055af98252fd3315)
---
 mm/zswap.c | 42 ++++++++++++++++++++++++------------------
 1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index db81fd7c3f1b2..7fefb2eb3fcd8 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -815,15 +815,15 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
 {
 	struct zswap_pool *pool = hlist_entry(node, struct zswap_pool, node);
 	struct crypto_acomp_ctx *acomp_ctx = per_cpu_ptr(pool->acomp_ctx, cpu);
-	struct crypto_acomp *acomp;
-	struct acomp_req *req;
+	struct crypto_acomp *acomp = NULL;
+	struct acomp_req *req = NULL;
+	u8 *buffer = NULL;
 	int ret;
 
-	mutex_lock(&acomp_ctx->mutex);
-	acomp_ctx->buffer = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));
-	if (!acomp_ctx->buffer) {
+	buffer = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));
+	if (!buffer) {
 		ret = -ENOMEM;
-		goto buffer_fail;
+		goto fail;
 	}
 
 	acomp = crypto_alloc_acomp_node(pool->tfm_name, 0, 0, cpu_to_node(cpu));
@@ -831,21 +831,25 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
 		pr_err("could not alloc crypto acomp %s : %ld\n",
 				pool->tfm_name, PTR_ERR(acomp));
 		ret = PTR_ERR(acomp);
-		goto acomp_fail;
+		goto fail;
 	}
-	acomp_ctx->acomp = acomp;
-	acomp_ctx->is_sleepable = acomp_is_async(acomp);
 
-	req = acomp_request_alloc(acomp_ctx->acomp);
+	req = acomp_request_alloc(acomp);
 	if (!req) {
 		pr_err("could not alloc crypto acomp_request %s\n",
 		       pool->tfm_name);
 		ret = -ENOMEM;
-		goto req_fail;
+		goto fail;
 	}
-	acomp_ctx->req = req;
 
+	/*
+	 * Only hold the mutex after completing allocations, otherwise we may
+	 * recurse into zswap through reclaim and attempt to hold the mutex
+	 * again resulting in a deadlock.
+	 */
+	mutex_lock(&acomp_ctx->mutex);
 	crypto_init_wait(&acomp_ctx->wait);
+
 	/*
 	 * if the backend of acomp is async zip, crypto_req_done() will wakeup
 	 * crypto_wait_req(); if the backend of acomp is scomp, the callback
@@ -854,15 +858,17 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
 	acomp_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG,
 				   crypto_req_done, &acomp_ctx->wait);
 
+	acomp_ctx->buffer = buffer;
+	acomp_ctx->acomp = acomp;
+	acomp_ctx->is_sleepable = acomp_is_async(acomp);
+	acomp_ctx->req = req;
 	mutex_unlock(&acomp_ctx->mutex);
 	return 0;
 
-req_fail:
-	crypto_free_acomp(acomp_ctx->acomp);
-acomp_fail:
-	kfree(acomp_ctx->buffer);
-buffer_fail:
-	mutex_unlock(&acomp_ctx->mutex);
+fail:
+	if (acomp)
+		crypto_free_acomp(acomp);
+	kfree(buffer);
 	return ret;
 }
 
-- 
2.48.0.rc2.279.g1de40edade-goog