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Received: from 30.74.144.191(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XDdS0nD_1793097334 cluster:ay36) by smtp.aliyun-inc.com; Tue, 27 Oct 2026 18:35:35 +0800 Message-ID: Date: Tue, 27 Oct 2026 18:35:34 +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> <87e32afd-75de-4407-8f0c-fb70b40d5eca@linux.alibaba.com> <81ee2c92-f342-4f7f-aacd-d9558e67a980@kernel.org> From: Baolin Wang In-Reply-To: <81ee2c92-f342-4f7f-aacd-d9558e67a980@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: E8877A0008 X-Stat-Signature: sxtegyxhpgrs657gsa1ehpxs1rygwutk 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: 1793097342-355663 X-HE-Meta: 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 U5Pkc6sb Rl7brhS3PBdsLzQW4I0QdMfN2UWD2hb4kpeFHbidY2PYC8RngvwtMefioYqvMUNH71Z9s40oC1xUsLYmTMr+WKbc4l9y/2MeUiAL61w4b6n8osSZqC2zcuUNcExRGMM7kINL3TnJyHyAtOancBatHxH1vh/2R03hj5Htcbc45XhECS4+G9VP+Bix2rNmkcs9Tnt7I5lDvcPqv3jFdcAT2+yW6fqHwHUWafI33ij2eS3rhFucRbbaAtTFy3mFnXKhkSrV/woYjHSs0OId6x9HmDY7hkMD3pAdbtpw34M/gquSgiHqO8aMxiXjWaGqtwpAHL/YC8hqTWZ9vIqNP8nmGhSfLqOgRtMicxFbaW2LorqcZ7PI84DxVVDPSMzrQCyxp/Zb8CtVvaM1CZotmMyj8i9aUE34UjJDe0VIf 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 5:33 PM, David Hildenbrand (Arm) wrote: > On 10/27/26 10:03, Baolin Wang wrote: >> >> >> 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] >> > > Best to wait to submit new versions if there are open discussions? Because now > we have the same discussion in two places. Sure. > > I think you're right about the race, the issue is that we check for > pmd_is_huge(), not pmd_present() which confused me. > > Clearest to me would be to not drop the second pmd_none() check but move it > where it belongs: if splitting zapped the PMD. > > @@ -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; > } IMHO, I'm not a big fan of adding two pmd_none() checks just to avoid an uncommon concurrent sync. If you don't like the approach in v4 [1] either, I'll follow your preferred coding style. @@ -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; } [1] https://lore.kernel.org/all/4cc521a118564b4145edf0324066ab532fb41181.1792979870.git.baolin.wang@linux.alibaba.com/#t > Removing the "if (pmd_none(*pmd)) {" in your variant is what confused me, > because it looks like we no longer happen the case where we zapped the PMD > ourselves. Because zap_pte_range() will return the same addr and check the none PMD again in zap_pmd_range() in this case, I thought you wanted this simple version, considering keeping two pmd_none() checks is incorrect.