From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 199D14E36F1 for ; Wed, 14 Oct 2026 13:25:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791984342; cv=none; b=NvRa9OCJBNanAfUdJaQgM6EBBwU1owEUYCPgBIXZkP0cYKXGXe27hgntbWTbdanidZuK14oomDg3m7wg2NFRCaIIz83kVRJ1bBOxQ9DOa9DPpHyt4pZIUV+rR7zMTNV2UAQWPQYJtV0KZryuVBIQdGTF8LqqSnvrDY4vhHenN44= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791984342; c=relaxed/simple; bh=9NwkIz9a3Ymm3b5eJuU0HgB2zY28SrOdGDMRPlG8Q5w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YCy+6y3FvTb7TDD9mquBQQFch/BrwfLSZPlpvzrnKuyfafj/z7FyYyTKm1lFn9OQcdBC6oNRwcDXg/Vg1MjYJZ6c1TcKnWD84H9ytGWKBIO8holSMKEXU/6yDjjR40TyA6jI+NxKit9BfQRUVcY2gL29HZI5aB8ojzy5tXe7IsE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=iPVJjUzD; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="iPVJjUzD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 61BC01F000FF; Wed, 14 Oct 2026 13:25:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791984340; bh=TQa+MBCQh2+NmFEKYE9b9rxt6YnvIpdiftCj4DjCvMU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=iPVJjUzDZq3OfFF3zZSXEbB4wZPD8Y4VDcWDLHR+NVFYm4OMfOCzjMTlqaxCxVzUk Pz81Th53bPT0DP2ykrJtnYjDImjIHN96Bdu5dmDBZtqa1oUovm3KDOAnR7j3bjEzur fh4c2SYFJLfdIsiXEKD754hwjYw2QfDG31C9+K6u2c26srf++dMn2GjvFps7Q9USJN fpENVIAMLtLx+JZZXO1oAwZ4ngFGa1Y+8YF7l2ryuYKLQp7exiQd7N23DsV86STq+q 3c0VQZdlV1hcXbXV7AHV83hAHpqBHrjXvrBI6haEx27lN2pCI1Xt00WRT6NtWp5Dtk Fx9ssbBpRRThw== Message-ID: <6660d3bb-3485-4c34-8734-3ea99df920f1@kernel.org> Date: Wed, 14 Oct 2026 15:25:33 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] 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: <542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com> <8245424a-f068-4be5-9e58-713cbff1f8ae@kernel.org> <405811f5-ee36-4b3c-80d0-7dc6827ab5d4@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 10/13/26 11:36, Baolin Wang wrote: > > > On 10/12/26 6:23 PM, David Hildenbrand (Arm) wrote: >> On 10/10/26 09:22, 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) >> >> That reminds me of sync_with_folio_pmd_zap() usage in page_vma_mapped_walk(). >> >> Isn't that the better approach? > > I had considered this approach before, but it is not feasible. IIUC, the > sync_with_folio_pmd_zap() is used to address the race with zap_huge_pmd(), and > both the PMD clear and the folio mapcount decrement are performed under the PMD > lock in zap_huge_pmd(). Just to be clear, I am thinking about something like the following on the zap_pmd_range() side: diff --git a/mm/internal.h b/mm/internal.h index 38b1165212c94..a8820ad7e2f21 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -584,14 +584,15 @@ 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() or zap_empty_pte_table() may have cleared the PMD but not + * yet modified the mapped folios to indicate they are unmapped. 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 8b0c2c735d3de..c585f5eb255c9 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2139,9 +2139,7 @@ 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)) { + } else if (details && details->single_folio && pmd_none(*pmd)) { sync_with_folio_pmd_zap(tlb->mm, pmd); } if (pmd_none(*pmd)) { > > However, the race here is different: tlb_flush_rmaps() is under the PTE lock, > while zap_empty_pte_table() is under the PMD lock, and reordering between the > two functions is also possible. That is, the folio mapcount operation could > execute after the acquire barrier (spin_lock()) of zap_empty_pte_table(), > allowing a reader on CPU 0 to incorrectly observe that the PMD is none while > mapcount > 0. I think we really just have to make sure to flush rmaps before zapping the PMD. Then it's really just the same thing we had before with sync_with_folio_pmd_zap(). So I agree about doing the tlb_flush_rmaps() before zapping the PMD. But on the zap_pmd_range() front that's really just what sync_with_folio_pmd_zap() wants to handle. -- Cheers, David