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Received: from 30.74.144.181(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XDX0EnS_1793012546 cluster:ay36) by smtp.aliyun-inc.com; Mon, 26 Oct 2026 19:02:26 +0800 Message-ID: <8779b778-c124-49ff-8072-2c995394ed8e@linux.alibaba.com> Date: Mon, 26 Oct 2026 19:02:25 +0800 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm: memory: fix truncation removing mapped folios To: "David Hildenbrand (Arm)" , akpm@linux-foundation.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, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <4cc521a118564b4145edf0324066ab532fb41181.1792979870.git.baolin.wang@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 54CBB40006 X-Stat-Signature: habia8rtmu1hoz337r3c84d4q4z97et4 X-Spam-Status: No, score=-24.8 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_SANE_1,USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=4.0.1 X-Rspam-User: X-HE-Tag: 1793012549-646837 X-HE-Meta: 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 ZSKRzy+b QRshY1HYsEXy4X/4ZeoZX3P5J86k8Xm+ZK07XAiMM8HGcQqA4SVv4SHasWzzzBdMd9rEr3ywDt/XIGj9aLbqJvoWHQOc2wBFRGarN8q+0lx35sLASltgrsXS/Iiqe7Kcyu0AKeyHKpBMzLGhgA5ujm3RkmW0OcpyYk7+YNbnceNCqxlKVtjsLa1p4D1tvu1T77kB8zlbV1d2WlapKhnFJSDW2k8kpD+VNvHvkRvhp6mERPrt7JRXB9hRZDIWUeFfV7gGFTdZxX3y+maTl6bsdAOC+QDcaJv7UQYIRyRacCOiTXhw33/fOunhzsjAND/jEvm528EnbeIEVYIqRSYIz+JZVzkuG+7rUHm+0Vahd59nEzeme/8eOuTWA0Enrjp2tZqVa 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 On 10/26/26 6:00 PM, David Hildenbrand (Arm) wrote: > On 10/26/26 03:05, Baolin Wang wrote: >> 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(). >> >> [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)") >> Acked-by: Qi Zheng >> Reviewed-by: Pedro Falcato >> Reviewed-by: Lorenzo Stoakes (ARM) >> Signed-off-by: Baolin Wang >> --- >> 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 | 28 +++++++++++++++------------- >> 2 files changed, 21 insertions(+), 17 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..6a93b7fa48b2 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); >> >> /* >> @@ -2102,12 +2105,11 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb, >> 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)) { >> + if (details && details->single_folio) >> + sync_with_folio_pmd_zap(tlb->mm, pmd); >> addr = next; >> continue; >> > Sorry if I sound annoyed, but why don't you keep the > > } else if (pmd_none(*pmd)) { > > above as I suggested? > > Because we fancy unnecessary PMD table lock cycles? Ah, sorry, I misunderstood what you meant before [1]. Yes, this is still correct and looks good to me. [1] https://lore.kernel.org/all/179c26ed-1764-4e5b-a7ee-047ddabbfea1@linux.alibaba.com/ Do you want me to respin a new version or just apply the following diff? diff --git a/mm/memory.c b/mm/memory.c index 6a93b7fa48b2..d9c54549b85b 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2105,9 +2105,7 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb, continue; } /* fall through */ - } - - if (pmd_none(*pmd)) { + } else if (pmd_none(*pmd)) { if (details && details->single_folio) sync_with_folio_pmd_zap(tlb->mm, pmd); addr = next;