[PATCH v2] mm: memory: fix truncation removing mapped folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Ayush reported an issue that shmem/tmpfs folios are still mapped when
truncating them on production:
"
BUG: Bad page cache in process ...  pfn:...
page dumped because: still mapped when deleted
"

I reproduced the issue with Ayush's reproducer [1] with the latest mm-unstable
branch, and I got the following crash info. From the dump message, we can
see that truncate_inode_folio() is really trying to remove mapped folios,
which is incorrect.

"
[  189.171977] page: refcount:3 mapcount:1 mapping:00000000fe168ee1
index:0x3880 pfn:0x191647
[  189.171995] memcg:ffff0000cc073d40
[  189.171997] aops:shmem_aops ino:3c01 dentry name(?):"memfd:runsc-memory"
[  189.172006] flags: 0x17fffef0002022d(locked|referenced|uptodate|lru|
workingset|swapbacked|node=0|zone=2|lastcpupid=0x3ffff)
.......
[  189.172017] page dumped because: VM_BUG_ON_FOLIO(folio_mapped(folio))
[  189.172026] ------------[ cut here ]------------
[  189.172027] kernel BUG at mm/filemap.c:155!
[  189.172057] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
.......
[  189.177721] CPU: 3 UID: 0 PID: 5521 Comm: repro Kdump: loaded Tainted:
7.3.0-rc4+ #277 PREEMPT(lazy)
.......
[  189.183986] Call trace:
[  189.184124]  filemap_unaccount_folio+0xf0/0x1e8 (P)
[  189.184391]  __filemap_remove_folio+0x34/0x160
[  189.184633]  filemap_remove_folio+0x4c/0xb0
[  189.184859]  truncate_inode_folio+0x34/0x58
[  189.185087]  shmem_undo_range+0x220/0x658
[  189.185308]  shmem_fallocate+0x2f0/0x470
[  189.185527]  vfs_fallocate+0x128/0x328
......
[  189.187078]  el0t_64_sync+0x184/0x188
"

After analysis, I believe the race exists between truncation and MADV_DONTNEED,
and shmem's ->map_pages() merely makes the issue easier to reproduce. Since
MADV_DONTNEED synchronously releases the pagetable page before calling
tlb_flush_rmaps(), this could cause another thread's truncation to skip
zap_pte_range() (due to pmd is none) but still observe the folio's mapcount
as non-zero. A possible race scenario is as follows:

CPU 0                           CPU 1
shmem_fallocate                 madvise_dontneed_single_vma
  unmap_mapping_range            .......
  ......
  (filemap_map_pages() remap folios)
				  zap_pte_range
  truncate_inode_folio		    zap_empty_pte_table(pmd clear)
    unmap_mapping_folio
    ......
      zap_pmd_range (saw pmd none, skip)

  filemap_remove_folio
    BUG_ON(folio_mapped)
                                    tlb_flush_rmaps(remove rmaps)

To fix this issue, we should move the pmd clear operation (via zap_empty_pte_table())
to after tlb_flush_rmaps() and add smp_wmb()/smp_rmb() memory barriers, so that
when we find a pmd_none() while unmapping a folio without holding the PTL, the
folio's mappings are guaranteed to have been removed via tlb_flush_rmaps().

[1] https://lore.kernel.org/all/20260925065013.3682431-1-ayushr@modal.com/
Reported-by: Ayush Ranjan <ayushr@modal.com>
Closes: https://lore.kernel.org/all/20260924061708.1645968-1-ayushr@modal.com/
Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
Changes from v1:
 - Add an smp_rmb() to pair with the smp_wmb() (Per Lorenzo and Pedro).
---
 mm/memory.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 6a8e7772b8d6..c3a1faf580c8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2036,6 +2036,15 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 		}
 	} while (pte += nr, addr += PAGE_SIZE * nr, addr != end);
 
+	add_mm_rss_vec(mm, rss);
+	lazy_mmu_mode_disable();
+
+	/* Do the actual TLB flush before dropping ptl */
+	if (force_flush) {
+		tlb_flush_mmu_tlbonly(tlb);
+		tlb_flush_rmaps(tlb, vma);
+	}
+
 	/*
 	 * Fast path: try to hold the pmd lock and unmap the PTE page.
 	 *
@@ -2044,16 +2053,13 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 	 * to ensure they are still none, thereby preventing the pte entries
 	 * from being repopulated by another thread.
 	 */
-	if (can_reclaim_pt && direct_reclaim && addr == end)
+	if (can_reclaim_pt && direct_reclaim && addr == end) {
+		/*
+		 * The rmap changes need to be observed before PTEs get zapped.
+		 * Pairs with smp_rmb() in zap_pmd_range().
+		 */
+		smp_wmb();
 		direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval);
-
-	add_mm_rss_vec(mm, rss);
-	lazy_mmu_mode_disable();
-
-	/* Do the actual TLB flush before dropping ptl */
-	if (force_flush) {
-		tlb_flush_mmu_tlbonly(tlb);
-		tlb_flush_rmaps(tlb, vma);
 	}
 	pte_unmap_unlock(start_pte, ptl);
 
@@ -2108,6 +2114,11 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
 			sync_with_folio_pmd_zap(tlb->mm, pmd);
 		}
 		if (pmd_none(*pmd)) {
+			/*
+			 * Order this PMD read before the folio's mapcount read.
+			 * Pairs with smp_wmb() in zap_pte_range().
+			 */
+			smp_rmb();
 			addr = next;
 			continue;
 		}
-- 
2.47.3