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Received: from localhost(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XDf2cRu_1793148781 cluster:ay36) by smtp.aliyun-inc.com; Wed, 28 Oct 2026 08:53:01 +0800 From: Baolin Wang To: akpm@linux-foundation.org, david@kernel.org Cc: ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, qi.zheng@linux.dev, jack@suse.cz, pfalcato@suse.de, ayushr@modal.com, baolin.wang@linux.alibaba.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH v6] mm: memory: fix truncation removing mapped folios Date: Wed, 28 Oct 2026 08:52:57 +0800 Message-ID: <14ebbd556cc4941f0ebc43da59fa6a2fdca25dc8.1793148609.git.baolin.wang@linux.alibaba.com> X-Mailer: git-send-email 2.43.5 X-Archived-By: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 4A9AE1C0007 X-Stat-Signature: 8mdkrjtb7grqui4751cbig5kpo4t9yfy X-Rspam-User: X-Spam-Status: No, score=-26.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_GIT,USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1793148787-863157 X-HE-Meta: U2FsdGVkX18cqoQve+3wwyIU2NYaYXVHPY0Q9unP5Im+0hCyBNzOIlGDbSOQosSiymIOuY66lZ1DpmLQxn6wMpsTAAYMww82w/6erISzMAL0glhmePLAYQU22fCyI7H7NGteB/dlNqmjTRM/Z9qQzBmsMyMlzIzqJtYHmdyTs2t8M6rcMbdYDxrY/Oew/mwfDH5mO/0Q/AaBcp9LXNne/EPTyGuJEAnJzwYc261x1OAwmNRFFmlMtyiwV33PA4pX2mX+07mxUs6CMQRjbwdj65sa/v9VMPa96+kbL2KGGVgEGjGGNazdpWgXAIBRAtNy4rg6kJlLhYO9A55UC+HW7r8d1b1KME/EqoVb17BgLE/tL2tNQWFNwuX/AXAkSTYZjLR9wsX6YGaL3eh9oHg48drWHLj3TyN7VBH3a8W7nFB7dj7ZWhVynTgZpVamgckcBdWRmhaPXjx43jxUtaclGEnIg8uUlxV/yfDWMjRQEoZRa0Td9GgpbA1KVHeAo7CeppfvX6r5G/qJ9FPwiqDKyoV+qHv/tGrpOoVIwRV5/bYYTWEOUbNMjphdGTwRdm2CY8hr1R70Z4pnn26s9BPKgDqoc7hakIrFCbjgey1Os4HKLnT/ArpuzVFc15xTgmW/IWEy0U35snvUBwvJE2yHHnwHbjrcvsOdAIUoA/F9cvgVT5dpCt/Qfs2Mqp8oSI7WlQJiPNJq027+FGkRkPuIIcLVRE53uXgTxIgGYRCtmMt4xqRO+/b6C/sLlk0uJPddTFJrZ458oJaliGij9MCBtC3W7+Jvu10L52LZ+gPZ944AfBcSDaD/ZxDR+FRgOz23oBoGvAG6P2BvuQn+LiLNME5dtjjkGu0sasNpBPSqflT8oMsTdsrxQdKnCeI0gFYAG1Cdf2wNAJVJsL8Yb3URtYwS9RAcxaDlt921z0fAAcKiDS+Fv6CE8gpylhUxtrIQbGmCrZyYAOZTc7uoTCc 2XwZ5R1g lZFjB3tQoEZCsGgPzFzZJ7qnDGMD+Mwz3RpkUB8rb9xOfw4ByFeQTsSKKBQWEwluPSsf28U8mDAi+pYHqmyHl9Ab9IJ67rj+EeRKZDDoz0NlrSKCJdvvsRUS66FhrGvZWqs8DP8mqmBT0810Qzt37oZKDSXwk2V2FUq5XybXYb8m9ESty0H2EKD+/jXLtpzZ52bdkSe2OD1uc7GoMqafMtkYVvhgaXUmZdWsdvuS4XHHybzqbyLXmv7uC9TWA3Ker2xKlPaVoVv7gulPDV8srvFgUvzkn6XeP5SBVUk7FtF6F7cGGhjOYnbmGtKZ5c5nvsiX9K9WGVzhWl9nTOVUi+6iGhA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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. [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)") Cc: stable@vger.kernel.org Acked-by: Qi Zheng Reviewed-by: Pedro Falcato Reviewed-by: Lorenzo Stoakes (ARM) Signed-off-by: Baolin Wang --- Andrew, sorry for the trouble caused by the several iterations:). Changes from v5: - Keep the 2nd pmd_none() check (per David). - Add Cc: stable (per Andrew). Changes from v4: - Put the pmd_none() check in the 'else if' branch (per David). Changes from v3: - Collect review tags from Qi, Pedro and Lorenzo. Thanks. - Update the related comments per Lorenzo. Changes from v2: - Use use sync_with_folio_pmd_zap() to sync with concurrent zapping (Per David). Changes from v1: - Add an smp_rmb() to pair with the smp_wmb() (Per Lorenzo and Pedro). --- mm/internal.h | 10 ++++++---- mm/memory.c | 35 ++++++++++++++++++++--------------- 2 files changed, 26 insertions(+), 19 deletions(-) diff --git a/mm/internal.h b/mm/internal.h index b1b0f29d2216..ffb8eaec90e3 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -584,14 +584,16 @@ 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 e911e63ba1ed..e0fef107a022 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. * @@ -2043,18 +2052,12 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, * 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); - - 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); /* @@ -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; } -- 2.47.3