linux-kernel - [PATCH] mm: zswap: move allocations during CPU init outside the lock

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Message-ID: <20250113214458.2123410-1-yosryahmed@google.com>
Date: Mon, 13 Jan 2025 21:44:58 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Yosry Ahmed <yosryahmed@...gle.com>
Subject: [PATCH] mm: zswap: move allocations during CPU init outside the lock

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.

Fixes: 12dcb0ef5406 ("mm: zswap: properly synchronize freeing resources during CPU hotunplug")
Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
---
 mm/zswap.c | 42 ++++++++++++++++++++++++------------------
 1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 30f5a27a68620..b84c20d889b1b 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -820,15 +820,15 @@ 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));
@@ -836,21 +836,25 @@ 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
@@ -859,15 +863,17 @@ 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;
 }
 
-- 
2.47.1.688.g23fc6f90ad-goog