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Received: from 30.74.144.191(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XDdD3Pp_1793091826 cluster:ay36) by smtp.aliyun-inc.com; Tue, 27 Oct 2026 17:03:47 +0800 Message-ID: <87e32afd-75de-4407-8f0c-fb70b40d5eca@linux.alibaba.com> Date: Tue, 27 Oct 2026 17:03:46 +0800 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5] 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: <2ba579e8-06fb-41f5-9eb2-f02b2aabe29b@kernel.org> From: Baolin Wang In-Reply-To: <2ba579e8-06fb-41f5-9eb2-f02b2aabe29b@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: A7191120007 X-Stat-Signature: he1jdr7ffja59swxzxzeinyyhfjauu4e X-Rspam-User: 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-HE-Tag: 1793091830-102044 X-HE-Meta: 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 r527yKFK Nky96XYD5AjEzT1H2QbCah3/GL7cH/hquXt1KFL9I6XdkpP0MCIICAH/p1VZHLxx8U7NZn4zZ7paC0ZtIfxDLcbwv009tMt0SLp7IHBbkqfVK+Vs+I4ahKyHKf7MYq699c+qKzro3hJO9p9pKAUD/jpp4vGfD5JSXnCBd1JIy9g+7YW8IFHj+0fp3mCWNDRh4f5jvyXOwMRpU0rPkElxDhxrnHseNGvdGLnBCRcHRvR7zLc9HXYgqFagwirA2kHtkQmyOrRMpPdd6SazJ2r5LHi5FDwiBCpz9e/loU3vei7U4Hbh/ZND3UvEjRdC9vzvkn4i6kTusR8ZH+imx9vpQUG1Hwvrsyk6WWrhECW3pfXCOPP0D/M7d3UFB9qlaUIhn9ldc 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 10/27/26 5:00 PM, David Hildenbrand (Arm) wrote: > On 10/27/26 03:19, 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 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 | 30 +++++++++++++++--------------- >> 2 files changed, 21 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..d9c54549b85b 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 +2105,9 @@ 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)) { >> + } else if (pmd_none(*pmd)) { >> + if (details && details->single_folio) >> + sync_with_folio_pmd_zap(tlb->mm, pmd); >> addr = next; >> continue; >> } > > > > } else if (pmd_none(*pmd) && details && details->single_folio)) { > sync_with_folio_pmd_zap(tlb->mm, pmd); > } > > if (pmd_none(*pmd) { > addr = next; > continue; > } > > > Minimal change required, no? No, keeping two pmd_none() checks cannot fix the race, as I replied before. [1] [1] https://lore.kernel.org/all/179c26ed-1764-4e5b-a7ee-047ddabbfea1@linux.alibaba.com/