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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Ka7GAHpt; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BD6E5606A31A for ; Wed, 28 Oct 2026 08:44:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Ka7GAHpt Received: by smtp.kernel.org (Postfix) with ESMTPSA id 813221F01825; Wed, 28 Oct 2026 08:44:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793177095; bh=f+44Zt08Ec+FbsUxCi2UdL40Z7OhmxVeqHmo7xhynXM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Ka7GAHptCX96Cpin6rN5Z+/BZuap2WQhtZisJyOT39uWcrkTSIbLyZSR4rlLQws7x 3Vi0sv2RQDBgdnEQOVD8q+UagGE7udWRxGX4IyNm1bgyK4pP3Wjsnn7nlyqhfdodiO mSVhnlg6HVWKg6M51eoLIQuCNqFAz9kTxhE2OUf93dl4Ejl2HZRzLXY5BgeIs9CXxT I4jjSOggK7qCzQGSwNtBvbB43FQ0SOaRIs8z3eOX1mrAr7qk3MCd/qxjG+wHyDPuhh GyHlIkilY8hvL3RXlzVllsvjSudkQ/a55la4UG6gP5ctAGG+B86KzDpsHtzCPiqFmd dsECo8easr2VA== Message-ID: <81862da7-21c7-4060-87f5-09650582ea24@kernel.org> Date: Wed, 28 Oct 2026 09:44:49 +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 v6] 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: <14ebbd556cc4941f0ebc43da59fa6a2fdca25dc8.1793148609.git.baolin.wang@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: 7bit On 10/28/26 01:52, 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(). > > Meanwhile move the second pmd_none() check to where it belongs: if splitting > zapped the PMD. > > [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)") > Cc: stable@vger.kernel.org > Acked-by: Qi Zheng > Reviewed-by: Pedro Falcato > Reviewed-by: Lorenzo Stoakes (ARM) > Signed-off-by: Baolin Wang > --- > Andrew, sorry for the trouble caused by the several iterations:). Acked-by: David Hildenbrand (Arm) Let's see what the fallout from this one will be (performance wise :) ) -- Cheers, David