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b=ExPbIF3/QgmyOpemsHOD0lOeZ/59J7ewh8pOXutB20lBVYY4W4lZSAcoQyIPY1bGx dUimDUTDPPtt83v1afbJycwVDKQGww5cP3aofRd/IwXMjzr/3fIVJq9U27IKgXsCfg Lz2nKWbFWvA9+KxdBDOIyi0pnPCtNE4d0lDXRJTAHHrZ9QjvKr/cQvexpJxrHyS5AN 916tZERFYOz9bgRP9rJ+/+uNo7m3xRtAKnLoJTROm951I7O3NRbGrlhdyVOAJohUmh upYM6MSGwKABZVMBA1CmXczakiVqMvKnMqeCyvLRTbZasGW5VwFlUd7uP8nXfFbVlF T2rIZVCs/OM4A== Message-ID: <81ee2c92-f342-4f7f-aacd-d9558e67a980@kernel.org> Date: Tue, 27 Oct 2026 10:33:22 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5] mm: memory: fix truncation removing mapped folios To: Baolin Wang , 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> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit 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. 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; } 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. -- Cheers, David