[PATCH 2/2] mm: avoid unnecessary PTE table lock during initial swap folio scan

[Wei Yang <richard.weiyang@gmail.com>]

The alloc_swap_folio() function performs an initial scan of the PTE
table solely to determine the potential size (order) of the folio
content that needs to be swapped in.

Locking the PTE table during this initial read is unnecessary for two
reasons:

  * We are not writing to the PTE table at this stage.

  * The code will re-check and lock the table again immediately before
    any actual modification is attempted.

This commit refactors the initial scan to map the PTE table without
acquiring the lock. This reduces contention and overhead, improving
performance of the swap-in path.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Chuanhua Han <hanchuanhua@oppo.com>
Cc: Barry Song <v-songbaohua@oppo.com>
---
 mm/memory.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 1b8ef4f0ea60..f8d6adfa83d7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4529,7 +4529,6 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
 	struct folio *folio;
 	unsigned long addr;
 	softleaf_t entry;
-	spinlock_t *ptl;
 	pte_t *pte;
 	gfp_t gfp;
 	int order;
@@ -4563,8 +4562,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
 	if (!orders)
 		goto fallback;
 
-	pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
-				  vmf->address & PMD_MASK, &ptl);
+	pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
 	if (unlikely(!pte))
 		return ERR_PTR(-EAGAIN);
 
@@ -4580,7 +4578,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
 		order = next_order(&orders, order);
 	}
 
-	pte_unmap_unlock(pte, ptl);
+	pte_unmap(pte);
 
 	/* Try allocating the highest of the remaining orders. */
 	gfp = vma_thp_gfp_mask(vma);
-- 
2.34.1