From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-112.freemail.mail.aliyun.com (out30-112.freemail.mail.aliyun.com [115.124.30.112]) (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 3F5963EB11E for ; Tue, 13 Oct 2026 09:36:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.112 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791884176; cv=none; b=GHQJbKjhTmtSB+sf4sPSKv7KKLwm+rr7CfQAylaxWwlgOmWJUrOph+J9Pg4YFrd56vpFtwC17E/mJKhKwItICVfdgFRqr5cYui4odjwDc69O4jljldxJ+7ugBva3yCgHh39Py4Kiz1IQjt8t+9JGYLwM1a1Smb8BPGavcnSTECE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791884176; c=relaxed/simple; bh=bSYBN0inOYfKRYiITpPuzhlLPrKLq5h/iogkgVYDOdo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=opUtxRNhCcRTdKmWSFwS8EFXmzzcFKvhU6WGBofFJrEGeDaCqQuDzmiqB0/rV0Eh7PgGUMQK8tvpBCjlNrLQbagdZxkcg1qU55CSte1yOtQr3TX4PmDOH6o1FP9mzdWMR8ejiIfj0zTEFEF8ETGzbEMfNBn4Wkb3TpSPJoDw8nw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=aAw0UjR0; arc=none smtp.client-ip=115.124.30.112 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="aAw0UjR0" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1791884171; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=tf91INvadMxOEv4mOizUmAwHCMyZFOULLoKUW7s5VsI=; b=aAw0UjR0Q00GkVKNB2lASq4k/GVXIlibJ/Ky8QNBqn+05nTDkKtfc/5NiGxIzF7fsysFNUXe9zzOY3ZftdlfnGKsb4Yg65kh1SrSzokIAfyaUteF0gjSXfrU55o9czwjts0FHoXl1bkl0uwnNvq9Z7DIW0yN844dahZcUBBBjaY= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045133197;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=14;SR=0;TI=SMTPD_---0XClKMcb_1791884168; Received: from 30.74.144.119(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XClKMcb_1791884168 cluster:ay36) by smtp.aliyun-inc.com; Tue, 13 Oct 2026 17:36:09 +0800 Message-ID: <405811f5-ee36-4b3c-80d0-7dc6827ab5d4@linux.alibaba.com> Date: Tue, 13 Oct 2026 17:36:08 +0800 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: "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: <542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com> <8245424a-f068-4be5-9e58-713cbff1f8ae@kernel.org> From: Baolin Wang In-Reply-To: <8245424a-f068-4be5-9e58-713cbff1f8ae@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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(). 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. Also, we need to consider the possible reordering on the reader side on CPU 0, as discussed earlier [1]. [1] https://lore.kernel.org/all/1b77c2d7-acc6-476a-b5cd-547471a46409@linux.alibaba.com/