linux-kernel - [PATCH 06/19] mm, swap: free the swap cache after folio is mapped

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Message-Id: <20251029-swap-table-p2-v1-6-3d43f3b6ec32@tencent.com>
Date: Wed, 29 Oct 2025 23:58:32 +0800
From: Kairui Song <ryncsn@...il.com>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>, 
 Barry Song <baohua@...nel.org>, Chris Li <chrisl@...nel.org>, 
 Nhat Pham <nphamcs@...il.com>, Johannes Weiner <hannes@...xchg.org>, 
 Yosry Ahmed <yosry.ahmed@...ux.dev>, David Hildenbrand <david@...hat.com>, 
 Youngjun Park <youngjun.park@....com>, Hugh Dickins <hughd@...gle.com>, 
 Baolin Wang <baolin.wang@...ux.alibaba.com>, 
 "Huang, Ying" <ying.huang@...ux.alibaba.com>, 
 Kemeng Shi <shikemeng@...weicloud.com>, 
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
 linux-kernel@...r.kernel.org, Kairui Song <kasong@...cent.com>
Subject: [PATCH 06/19] mm, swap: free the swap cache after folio is mapped

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

To prevent repeated faults of parallel swapin of the same PTE, remove
the folio from the swap cache after the folio is mapped. So any user
faulting from the swap PTE should see the folio in the swap cache and
wait on it.

Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/memory.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 6c5cd86c4a66..589d6fc3d424 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4362,6 +4362,7 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
 static inline bool should_try_to_free_swap(struct swap_info_struct *si,
 					   struct folio *folio,
 					   struct vm_area_struct *vma,
+					   unsigned int extra_refs,
 					   unsigned int fault_flags)
 {
 	if (!folio_test_swapcache(folio))
@@ -4384,7 +4385,7 @@ static inline bool should_try_to_free_swap(struct swap_info_struct *si,
 	 * reference only in case it's likely that we'll be the exclusive user.
 	 */
 	return (fault_flags & FAULT_FLAG_WRITE) && !folio_test_ksm(folio) &&
-		folio_ref_count(folio) == (1 + folio_nr_pages(folio));
+		folio_ref_count(folio) == (extra_refs + folio_nr_pages(folio));
 }
 
 static vm_fault_t pte_marker_clear(struct vm_fault *vmf)
@@ -4935,15 +4936,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	 */
 	arch_swap_restore(folio_swap(entry, folio), folio);
 
-	/*
-	 * Remove the swap entry and conditionally try to free up the swapcache.
-	 * We're already holding a reference on the page but haven't mapped it
-	 * yet.
-	 */
-	swap_free_nr(entry, nr_pages);
-	if (should_try_to_free_swap(si, folio, vma, vmf->flags))
-		folio_free_swap(folio);
-
 	add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
 	add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
 	pte = mk_pte(page, vma->vm_page_prot);
@@ -4997,6 +4989,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	arch_do_swap_page_nr(vma->vm_mm, vma, address,
 			pte, pte, nr_pages);
 
+	/*
+	 * Remove the swap entry and conditionally try to free up the
+	 * swapcache. Do it after mapping so any raced page fault will
+	 * see the folio in swap cache and wait for us.
+	 */
+	swap_free_nr(entry, nr_pages);
+	if (should_try_to_free_swap(si, folio, vma, nr_pages, vmf->flags))
+		folio_free_swap(folio);
+
 	folio_unlock(folio);
 	if (unlikely(folio != swapcache)) {
 		/*

-- 
2.51.1