From: Josef Bacik MGLRU passes stack copies of page-table entries in walk_pud_range() and walk_pmd_range() when walking to the next level. These copies can become stale when khugepaged or MADV_DONTNEED (CONFIG_PT_RECLAIM) removes a PTE table concurrently. This pattern dates back to commit bd74fdaea146 ("mm: multi-gen LRU: support page table walks"). Likely, earlier locking masked the latent bug. Later changes exposed it: commit 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock") allowed concurrent retraction, and commit cf4a65539c13 ("memcg-v1: no need for memcg locking for MGLRU") dropped the walker's outer RCU protection. The fix is simple, so just fix the original commit so we don't have to speculate. To fix it, pass the actual PMD/PUD pointers. This restores the RCU-protected lookup and validation against the live PMD. Note that the PUD handling is required as well to handle folded page tables correctly in all cases. Fix generated by an LLM and sent by Josef Bacik in private. [ david: handle walk_pud_range() as well, clarify+simplify description, adjust Fixes: ] Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks") Cc: stable@vger.kernel.org Signed-off-by: Josef Bacik Reviewed-by: Rik van Riel Reviewed-by: Qi Zheng Reviewed-by: Dev Jain Reviewed-by: Lorenzo Stoakes (ARM) Reviewed-by: Kairui Song Tested-by: Muhammad Usama Anjum Reviewed-by: Harry Yoo (Meta) Reviewed-by: Barry Song Reviewed-by: Baolin Wang Reviewed-by: Pedro Falcato Reviewed-by: Kiryl Shutsemau (Meta) Reviewed-by: SJ Park Signed-off-by: David Hildenbrand (Arm) --- mm/vmscan.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 9959ca158ce83..6a1af9b92349f 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3911,7 +3911,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end, walk->mm_stats[MM_NONLEAF_FOUND]++; - if (!walk_pte_range(&val, addr, next, args)) + if (!walk_pte_range(pmd + i, addr, next, args)) continue; walk->mm_stats[MM_NONLEAF_ADDED]++; @@ -3947,7 +3947,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long start, unsigned long end, if (!pud_present(val) || WARN_ON_ONCE(pud_leaf(val))) continue; - walk_pmd_range(&val, addr, next, args); + walk_pmd_range(pud + i, addr, next, args); if (need_resched() || walk->batched >= MAX_LRU_BATCH) { end = (addr | ~PUD_MASK) + 1; -- 2.43.0