[PATCH v3 04/15] mm, swap: check page poison flag after locking it

Kairui Song
2025-09-10T16:09:10Z

From: Kairui Song <kasong@tencent.com>

Instead of checking the poison flag only in the fast swap cache lookup
path, always check the poison flags after locking a swap cache folio.

There are two reasons to do so.

The folio is unstable and could be removed from the swap cache anytime,
so it's totally possible that the folio is no longer the backing folio
of a swap entry, and could be an irrelevant poisoned folio. We might
mistakenly kill a faulting process.

And it's totally possible or even common for the slow swap in path
(swapin_readahead) to bring in a cached folio. The cache folio could be
poisoned, too. Only checking the poison flag in the fast path will miss
such folios.

The race window is tiny, so it's very unlikely to happen, though.
While at it, also add a unlikely prefix.

Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
---
mm/memory.c | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 10ef528a5f44..94a5928e8ace 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4661,10 +4661,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
goto out;

folio = swap_cache_get_folio(entry);
- if (folio) {
+ if (folio)
swap_update_readahead(folio, vma, vmf->address);
- page = folio_file_page(folio, swp_offset(entry));
- }
swapcache = folio;

if (!folio) {
@@ -4735,20 +4733,13 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
ret = VM_FAULT_MAJOR;
count_vm_event(PGMAJFAULT);
count_memcg_event_mm(vma->vm_mm, PGMAJFAULT);
- page = folio_file_page(folio, swp_offset(entry));
- } else if (PageHWPoison(page)) {
- /*
- * hwpoisoned dirty swapcache pages are kept for killing
- * owner processes (which may be unknown at hwpoison time)
- */
- ret = VM_FAULT_HWPOISON;
- goto out_release;
}

ret |= folio_lock_or_retry(folio, vmf);
if (ret & VM_FAULT_RETRY)
goto out_release;

+ page = folio_file_page(folio, swp_offset(entry));
if (swapcache) {
/*
* Make sure folio_free_swap() or swapoff did not release the
@@ -4761,6 +4752,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
page_swap_entry(page).val != entry.val))
goto out_page;

+ if (unlikely(PageHWPoison(page))) {
+ /*
+ * hwpoisoned dirty swapcache pages are kept for killing
+ * owner processes (which may be unknown at hwpoison time)
+ */
+ ret = VM_FAULT_HWPOISON;
+ goto out_page;
+ }
+
/*
* KSM sometimes has to copy on read faults, for example, if
* folio->index of non-ksm folios would be nonlinear inside the
--
2.51.0