[PATCH 6.1.y] mm/mglru: skip special VMAs in lru_gen_look_around()

[Jakov Novak <jakovnovak30@gmail.com>]

From: Yu Zhao <yuzhao@google.com>

[ Upstream commit c28ac3c7eb945fee6e20f47d576af68fdff1392a ]

Special VMAs like VM_PFNMAP can contain anon pages from COW.  There isn't
much profit in doing lookaround on them.  Besides, they can trigger the
pte_special() warning in get_pte_pfn().

Skip them in lru_gen_look_around().

Link: https://lkml.kernel.org/r/20231223045647.1566043-1-yuzhao@google.com
Fixes: 018ee47f1489 ("mm: multi-gen LRU: exploit locality in rmap")
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reported-by: syzbot+03fd9b3f71641f0ebf2d@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/000000000000f9ff00060d14c256@google.com/
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[fix conflicts with variable declarations and vma pointer usage]
Signed-off-by: Jakov Novak <jakovnovak30@gmail.com>
---
 mm/vmscan.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1f7a90ecc700..f6f8c18dc45f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4622,6 +4622,7 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 	struct lru_gen_mm_walk *walk;
 	int young = 0;
 	unsigned long bitmap[BITS_TO_LONGS(MIN_LRU_BATCH)] = {};
+	struct vm_area_struct *vma = pvmw->vma;
 	struct folio *folio = pfn_folio(pvmw->pfn);
 	struct mem_cgroup *memcg = folio_memcg(folio);
 	struct pglist_data *pgdat = folio_pgdat(folio);
@@ -4635,11 +4636,15 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 	if (spin_is_contended(pvmw->ptl))
 		return;
 
+	/* exclude special VMAs containing anon pages from COW */
+	if (vma->vm_flags & VM_SPECIAL)
+		return;
+
 	/* avoid taking the LRU lock under the PTL when possible */
 	walk = current->reclaim_state ? current->reclaim_state->mm_walk : NULL;
 
-	start = max(pvmw->address & PMD_MASK, pvmw->vma->vm_start);
-	end = min(pvmw->address | ~PMD_MASK, pvmw->vma->vm_end - 1) + 1;
+	start = max(pvmw->address & PMD_MASK, vma->vm_start);
+	end = min(pvmw->address | ~PMD_MASK, vma->vm_end - 1) + 1;
 
 	if (end - start > MIN_LRU_BATCH * PAGE_SIZE) {
 		if (pvmw->address - start < MIN_LRU_BATCH * PAGE_SIZE / 2)
@@ -4660,7 +4665,7 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 	for (i = 0, addr = start; addr != end; i++, addr += PAGE_SIZE) {
 		unsigned long pfn;
 
-		pfn = get_pte_pfn(pte[i], pvmw->vma, addr);
+		pfn = get_pte_pfn(pte[i], vma, addr);
 		if (pfn == -1)
 			continue;
 
@@ -4671,7 +4676,7 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 		if (!folio)
 			continue;
 
-		if (!ptep_test_and_clear_young(pvmw->vma, addr, pte + i))
+		if (!ptep_test_and_clear_young(vma, addr, pte + i))
 			VM_WARN_ON_ONCE(true);
 
 		young++;
-- 
2.54.0