linux-kernel - [PATCH 3/7] mm, swap: avoid redundant swap device pinning

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Message-ID: <20250214175709.76029-4-ryncsn@gmail.com>
Date: Sat, 15 Feb 2025 01:57:05 +0800
From: Kairui Song <ryncsn@...il.com>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Chris Li <chrisl@...nel.org>,
	Barry Song <v-songbaohua@...o.com>,
	Hugh Dickins <hughd@...gle.com>,
	Yosry Ahmed <yosryahmed@...gle.com>,
	"Huang, Ying" <ying.huang@...ux.alibaba.com>,
	Baoquan He <bhe@...hat.com>,
	Nhat Pham <nphamcs@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Kalesh Singh <kaleshsingh@...gle.com>,
	linux-kernel@...r.kernel.org,
	Kairui Song <kasong@...cent.com>
Subject: [PATCH 3/7] mm, swap: avoid redundant swap device pinning

From: Kairui Song <kasong@...cent.com>

There are only two callers of __read_swap_cache_async not holding a swap
device reference, so make them hold a reference instead, and drop the
get/put_swap_device calls in __read_swap_cache_async. This should reduce
the overhead for swap in during page fault slightly.

Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/swap_state.c | 14 ++++++++------
 mm/zswap.c      |  6 ++++++
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index a54b035d6a6c..50840a2887a5 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -426,17 +426,13 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		struct mempolicy *mpol, pgoff_t ilx, bool *new_page_allocated,
 		bool skip_if_exists)
 {
-	struct swap_info_struct *si;
+	struct swap_info_struct *si = swp_swap_info(entry);
 	struct folio *folio;
 	struct folio *new_folio = NULL;
 	struct folio *result = NULL;
 	void *shadow = NULL;
 
 	*new_page_allocated = false;
-	si = get_swap_device(entry);
-	if (!si)
-		return NULL;
-
 	for (;;) {
 		int err;
 		/*
@@ -532,7 +528,6 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 	put_swap_folio(new_folio, entry);
 	folio_unlock(new_folio);
 put_and_return:
-	put_swap_device(si);
 	if (!(*new_page_allocated) && new_folio)
 		folio_put(new_folio);
 	return result;
@@ -552,11 +547,16 @@ struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		struct vm_area_struct *vma, unsigned long addr,
 		struct swap_iocb **plug)
 {
+	struct swap_info_struct *si;
 	bool page_allocated;
 	struct mempolicy *mpol;
 	pgoff_t ilx;
 	struct folio *folio;
 
+	si = get_swap_device(entry);
+	if (!si)
+		return NULL;
+
 	mpol = get_vma_policy(vma, addr, 0, &ilx);
 	folio = __read_swap_cache_async(entry, gfp_mask, mpol, ilx,
 					&page_allocated, false);
@@ -564,6 +564,8 @@ struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 
 	if (page_allocated)
 		swap_read_folio(folio, plug);
+
+	put_swap_device(si);
 	return folio;
 }
 
diff --git a/mm/zswap.c b/mm/zswap.c
index ac9d299e7d0c..83dfa1f9e689 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1051,14 +1051,20 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	struct folio *folio;
 	struct mempolicy *mpol;
 	bool folio_was_allocated;
+	struct swap_info_struct *si;
 	struct writeback_control wbc = {
 		.sync_mode = WB_SYNC_NONE,
 	};
 
 	/* try to allocate swap cache folio */
+	si = get_swap_device(swpentry);
+	if (!si)
+		return -EEXIST;
+
 	mpol = get_task_policy(current);
 	folio = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
 				NO_INTERLEAVE_INDEX, &folio_was_allocated, true);
+	put_swap_device(si);
 	if (!folio)
 		return -ENOMEM;
 
-- 
2.48.1