From 243cf1f67b365668389832463fc31cbb9fec47af Mon Sep 17 00:00:00 2001 From: Baolin Wang Date: Wed, 28 Oct 2026 08:52:57 +0800 Subject: [PATCH] mm: memory: fix truncation removing mapped folios 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 use sync_with_folio_pmd_zap() to sync with concurrent zapping, 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(). Meanwhile move the second pmd_none() check to where it belongs: if splitting zapped the PMD. Link: https://lore.kernel.org/14ebbd556cc4941f0ebc43da59fa6a2fdca25dc8.1793148609.git.baolin.wang@linux.alibaba.com Link: https://lore.kernel.org/all/20260925065013.3682431-1-ayushr@modal.com/ [1] Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)") Signed-off-by: Baolin Wang Signed-off-by: Andrew Morton Reported-by: Ayush Ranjan Closes: https://lore.kernel.org/all/20260924061708.1645968-1-ayushr@modal.com/ Acked-by: Qi Zheng Reviewed-by: Pedro Falcato Reviewed-by: Lorenzo Stoakes (ARM) Acked-by: David Hildenbrand (Arm) Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Mike Rapoport Cc: Suren Baghdasaryan Cc: Michal Hocko Cc: Jan Kara Cc: --- mm/internal.h | 10 ++++++---- mm/memory.c | 39 ++++++++++++++++++++++----------------- 2 files changed, 28 insertions(+), 21 deletions(-) diff --git a/mm/internal.h b/mm/internal.h index 566cbd2dce48..8b1263e81054 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -589,14 +589,16 @@ void free_pgtables(struct mmu_gather *tlb, struct unmap_desc *desc); void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte); /** - * sync_with_folio_pmd_zap - sync with concurrent zapping of a folio PMD + * sync_with_folio_pmd_zap - sync with concurrent zapping of folios * @mm: The mm_struct. * @pmdp: Pointer to the pmd that was found to be pmd_none(). * * When we find a pmd_none() while unmapping a folio without holding the PTL, - * zap_huge_pmd() may have cleared the PMD but not yet modified the folio to - * indicate that it's unmapped. Skipping the PMD without synchronization could - * make folio unmapping code assume that unmapping failed. + * zap_huge_pmd() may have cleared the PMD but not yet updated the folio + * mapcount, and zap_empty_pte_table() clears the PMD only after the rmaps + * for the table have been removed, but nothing orders the pmd_none() read + * against later mapcount reads. Skipping the PMD without synchronization + * could make folio unmapping code assume that unmapping failed. * * Wait for concurrent zapping to complete by grabbing the PTL. */ diff --git a/mm/memory.c b/mm/memory.c index 591592bc5a94..916637e8381d 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2036,17 +2036,6 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, } } while (pte += nr, addr += PAGE_SIZE * nr, addr != end); - /* - * Fast path: try to hold the pmd lock and unmap the PTE page. - * - * If the pte lock was released midway (retry case), or if the attempt - * to hold the pmd lock failed, then we need to recheck all pte entries - * 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) - direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval); - add_mm_rss_vec(mm, rss); lazy_mmu_mode_disable(); @@ -2055,6 +2044,20 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, tlb_flush_mmu_tlbonly(tlb); tlb_flush_rmaps(tlb, vma); } + + /* + * Fast path: try to hold the pmd lock and unmap the PTE page. + * + * If the pte lock was released midway (retry case), or if the attempt + * to hold the pmd lock failed, then we need to recheck all pte entries + * to ensure they are still none, thereby preventing the pte entries + * from being repopulated by another thread. + * + * Clear the PMD after the rmaps for the table have been removed, in case + * an incorrect mapcount is observed after unmapping the folio. + */ + if (can_reclaim_pt && direct_reclaim && addr == end) + direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval); pte_unmap_unlock(start_pte, ptl); /* @@ -2101,13 +2104,15 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb, addr = next; continue; } + /* We already synchronized with concurrent zapping. */ + if (pmd_none(*pmd)) { + addr = next; + continue; + } /* fall through */ - } else if (details && details->single_folio && - folio_test_pmd_mappable(details->single_folio) && - next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) { - sync_with_folio_pmd_zap(tlb->mm, pmd); - } - if (pmd_none(*pmd)) { + } else if (pmd_none(*pmd)) { + if (details && details->single_folio) + sync_with_folio_pmd_zap(tlb->mm, pmd); addr = next; continue; }