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 a smp_wmb() memory barrier, 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 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 --- mm/memory.c | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index 1f5d5f7d39cd..0272217ad7b0 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 +2053,10 @@ 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) { + /* rmap changes need to be observed before e.g PTEs get zapped. */ + 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); -- 2.47.3