[RFC 2/3] mm: process_mrelease: skip LRU movement for exclusive file folios

[Minchan Kim]

For the process_mrelease reclaim, skip LRU handling for exclusive
file-backed folios since they will be freed soon so pointless
to move around in the LRU.

This avoids costly LRU movement which accounts for a significant portion
of the time during unmap_page_range.

-   91.31%     0.00%  mmap_exit_test   [kernel.kallsyms]  [.] exit_mm
     exit_mm
     __mmput
     exit_mmap
     unmap_vmas
   - unmap_page_range
      - 55.75% folio_mark_accessed
         + 48.79% __folio_batch_add_and_move
           4.23% workingset_activation
      + 12.94% folio_remove_rmap_ptes
      + 9.86% page_table_check_clear
      + 3.34% tlb_flush_mmu
        1.06% __page_table_check_pte_clear

Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/memory.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 2f815a34d924..25e17893c919 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1640,6 +1640,8 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
 	bool delay_rmap = false;
 
 	if (!folio_test_anon(folio)) {
+		bool skip_mark_accessed;
+
 		ptent = get_and_clear_full_ptes(mm, addr, pte, nr, tlb->fullmm);
 		if (pte_dirty(ptent)) {
 			folio_mark_dirty(folio);
@@ -1648,7 +1650,16 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
 				*force_flush = true;
 			}
 		}
-		if (pte_young(ptent) && likely(vma_has_recency(vma)))
+
+		/*
+		 * For the process_mrelease reclaim, skip LRU handling for exclusive
+		 * file-backed folios since they will be freed soon so pointless
+		 * to move around in the LRU.
+		 */
+		skip_mark_accessed = mm_flags_test(MMF_UNSTABLE, mm) &&
+				     folio_mapcount(folio) < 2;
+		if (likely(!skip_mark_accessed) && pte_young(ptent) &&
+		    likely(vma_has_recency(vma)))
 			folio_mark_accessed(folio);
 		rss[mm_counter(folio)] -= nr;
 	} else {
-- 
2.54.0.rc0.605.g598a273b03-goog