[PATCH 1/2] mm: bail out do_swap_page() when no PTE table exist

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

The alloc_swap_folio() function scans the PTE table to determine the
potential size (order) of the folio content to be swapped in.

Currently, if the call to pte_offset_map_lock() returns NULL, it
indicates that no PTE table exists for that range. Despite this, the
code proceeds to allocate an order-0 folio and continues the swap-in
process. This is unnecessary if the required table is absent.

This commit modifies the logic to immediately bail out of the swap-in
process when the PTE table is missing (i.e., pte_offset_map_lock()
returns NULL). This ensures we do not attempt to continue swapping when
the page table structure is incomplete or changed, preventing
unnecessary work.

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 | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 2a55edc48a65..1b8ef4f0ea60 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4566,7 +4566,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
 	pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
 				  vmf->address & PMD_MASK, &ptl);
 	if (unlikely(!pte))
-		goto fallback;
+		return ERR_PTR(-EAGAIN);
 
 	/*
 	 * For do_swap_page, find the highest order where the aligned range is
@@ -4709,6 +4709,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 		    __swap_count(entry) == 1) {
 			/* skip swapcache */
 			folio = alloc_swap_folio(vmf);
+			if (IS_ERR(folio))
+				goto out;
 			if (folio) {
 				__folio_set_locked(folio);
 				__folio_set_swapbacked(folio);
-- 
2.34.1