[PATCH v2 1/4] mm: Set folio swapbacked iff folios are dirty in try_to_unmap_one

Barry Song
2025-01-13T03:42:11Z

From: Barry Song <v-songbaohua@oppo.com>

The refcount may be temporarily or long-term increased, but this does
not change the fundamental nature of the folio already being lazy-
freed. Therefore, we only reset 'swapbacked' when we are certain the
folio is dirty and not droppable.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
mm/rmap.c | 49 ++++++++++++++++++++++---------------------------
1 file changed, 22 insertions(+), 27 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index c6c4d4ea29a7..de6b8c34e98c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1868,34 +1868,29 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
*/
smp_rmb();

- /*
- * The only page refs must be one from isolation
- * plus the rmap(s) (dropped by discard:).
- */
- if (ref_count == 1 + map_count &&
- (!folio_test_dirty(folio) ||
- /*
- * Unlike MADV_FREE mappings, VM_DROPPABLE
- * ones can be dropped even if they've
- * been dirtied.
- */
- (vma->vm_flags & VM_DROPPABLE))) {
- dec_mm_counter(mm, MM_ANONPAGES);
- goto discard;
- }
-
- /*
- * If the folio was redirtied, it cannot be
- * discarded. Remap the page to page table.
- */
- set_pte_at(mm, address, pvmw.pte, pteval);
- /*
- * Unlike MADV_FREE mappings, VM_DROPPABLE ones
- * never get swap backed on failure to drop.
- */
- if (!(vma->vm_flags & VM_DROPPABLE))
+ if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
+ /*
+ * redirtied either using the page table or a previously
+ * obtained GUP reference.
+ */
+ set_pte_at(mm, address, pvmw.pte, pteval);
folio_set_swapbacked(folio);
- goto walk_abort;
+ goto walk_abort;
+ } else if (ref_count != 1 + map_count) {
+ /*
+ * Additional reference. Could be a GUP reference or any
+ * speculative reference. GUP users must mark the folio
+ * dirty if there was a modification. This folio cannot be
+ * reclaimed right now either way, so act just like nothing
+ * happened.
+ * We'll come back here later and detect if the folio was
+ * dirtied when the additional reference is gone.
+ */
+ set_pte_at(mm, address, pvmw.pte, pteval);
+ goto walk_abort;
+ }
+ dec_mm_counter(mm, MM_ANONPAGES);
+ goto discard;
}

if (swap_duplicate(entry) < 0) {
--
2.39.3 (Apple Git-146)