mm/mglru: skip special VMAs in lru_gen_look_around()

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: 018ee47f ("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>

mm/vmscan.c

@@ -3955,6 +3955,7 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 	int young = 0;
 	pte_t *pte = pvmw->pte;
 	unsigned long addr = pvmw->address;
+	struct vm_area_struct *vma = pvmw->vma;
 	struct folio *folio = pfn_folio(pvmw->pfn);
 	bool can_swap = !folio_is_file_lru(folio);
 	struct mem_cgroup *memcg = folio_memcg(folio);
@@ -3969,11 +3970,15 @@ 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(addr & PMD_MASK, pvmw->vma->vm_start);
-	end = min(addr | ~PMD_MASK, pvmw->vma->vm_end - 1) + 1;
+	start = max(addr & PMD_MASK, vma->vm_start);
+	end = min(addr | ~PMD_MASK, vma->vm_end - 1) + 1;
 
 	if (end - start > MIN_LRU_BATCH * PAGE_SIZE) {
 		if (addr - start < MIN_LRU_BATCH * PAGE_SIZE / 2)
@@ -3998,7 +4003,7 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 		unsigned long pfn;
 		pte_t ptent = ptep_get(pte + i);
 
-		pfn = get_pte_pfn(ptent, pvmw->vma, addr);
+		pfn = get_pte_pfn(ptent, vma, addr);
 		if (pfn == -1)
 			continue;
 
@@ -4009,7 +4014,7 @@ 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++;