linux-kernel - [PATCH 4/5] swap: remove the swap lock in swap_cache_get

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Date:   Fri,  9 Dec 2022 02:02:08 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Kairui Song <kasong@...cent.com>
Subject: [PATCH 4/5] swap: remove the swap lock in swap_cache_get_folio

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

There is only one caller not keep holding a reference or lock the
swap device while calling this function. Just move the lock out
of this function, it only used to prevent swapoff, and this helper
function is very short so there is no performance regression
issue. Help saves a few cycles.

Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/shmem.c      | 8 +++++++-
 mm/swap_state.c | 8 ++------
 2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index c1d8b8a1aa3b..0183b6678270 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1725,6 +1725,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	struct address_space *mapping = inode->i_mapping;
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct mm_struct *charge_mm = vma ? vma->vm_mm : NULL;
+	struct swap_info_struct *si;
 	struct folio *folio = NULL;
 	swp_entry_t swap;
 	int error;
@@ -1737,7 +1738,12 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 		return -EIO;
 
 	/* Look it up and read it in.. */
-	folio = swap_cache_get_folio(swap, NULL, 0);
+	si = get_swap_device(swap);
+	if (si) {
+		folio = swap_cache_get_folio(swap, NULL, 0);
+		put_swap_device(si);
+	}
+
 	if (!folio) {
 		/* Or update major stats only when swapin succeeds?? */
 		if (fault_type) {
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 19089417abd1..eba388f67741 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -324,19 +324,15 @@ static inline bool swap_use_vma_readahead(void)
  * unlocked and with its refcount incremented - we rely on the kernel
  * lock getting page table operations atomic even if we drop the folio
  * lock before returning.
+ *
+ * Caller must lock the swap device or hold a reference to keep it valid.
  */
 struct folio *swap_cache_get_folio(swp_entry_t entry,
 		struct vm_area_struct *vma, unsigned long addr)
 {
 	struct folio *folio;
-	struct swap_info_struct *si;
 
-	si = get_swap_device(entry);
-	if (!si)
-		return NULL;
 	folio = filemap_get_folio(swap_address_space(entry), swp_offset(entry));
-	put_swap_device(si);
-
 	if (folio) {
 		bool vma_ra = swap_use_vma_readahead();
 		bool readahead;
-- 
2.35.2