From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-118.freemail.mail.aliyun.com (out30-118.freemail.mail.aliyun.com [115.124.30.118]) (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 4B947356A38 for ; Sat, 17 Oct 2026 05:03:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.118 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792213434; cv=none; b=l8SGSsFkzdTGVj2ve1QxeGL3NVhghWFk5r/94gdq6EQVFIb+3I6i+DRUf5yHCua/UIBTLRAwlD6GjG6ZM6XjUV0eCgZgnwn58N+fkdEaxxIsEgcnOXwW20juIHInc2xcDOBmrkiYdrwKlsGhrTzKnSN8kBHiBd8OjZDkilby9o0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792213434; c=relaxed/simple; bh=SOv12bKZ/oM9WsZs+pnaxa0RyeoxWC/41oXvgZAmAkw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QiXXBBSaRgWrW/48JO2SKvzNSeHrTDq1qseL+GnhyhhqePCH8gvQz2LeD09h3RkSXttm2QCNpcsSZYHzskTSCc+Tjxuts7pFh69DYXWE6djt+fZzGbtcSrpJ66thUZeoAs6UwUtHgVRfEh+g1OVvcFKXwvMgbT9zYOAT9P1exzo= 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=IjwlADOE; arc=none smtp.client-ip=115.124.30.118 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="IjwlADOE" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1792213422; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=3bNNdf54bPSOyu30QFMPlpmkgCZp3aoSZH9DnyMrUgk=; b=IjwlADOExBnBIWxw5d6Bwq6SBlQ+1meCQkT6qmmzuu3J6SqZp3Y+neClfsHVlakQV9fci1odUrb+4lSPbPnm5UjYWf0NQUlH6V+sKHHgJYPU2KUzCYvgp2g+Yin6FApAxJwdlvfINXqyKzlS4ZnsTl5NIrPMHHrVIEKwqScMZu0= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R691e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045098064;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=14;SR=0;TI=SMTPD_---0XCwamdB_1792213420; Received: from 30.42.72.158(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XCwamdB_1792213420 cluster:ay36) by smtp.aliyun-inc.com; Sat, 17 Oct 2026 13:03:40 +0800 Message-ID: <21710bdf-3923-4e09-a99c-db50bf7e3247@linux.alibaba.com> Date: Sat, 17 Oct 2026 13:03:39 +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 v3] mm: memory: fix truncation removing mapped folios To: "Lorenzo Stoakes (ARM)" Cc: akpm@linux-foundation.org, david@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: From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/17/26 1:36 AM, Lorenzo Stoakes (ARM) wrote: > On Fri, Oct 16, 2026 at 09:48:08AM +0800, 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)") >> Signed-off-by: Baolin Wang > > With nits addressed LGTM, so: > > Reviewed-by: Lorenzo Stoakes (ARM) Thanks. > >> --- >> 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 | 9 +++++---- >> mm/memory.c | 25 ++++++++++++------------- >> 2 files changed, 17 insertions(+), 17 deletions(-) >> >> diff --git a/mm/internal.h b/mm/internal.h >> index 0434dfcfc36f..6d8f494296a4 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -585,14 +585,15 @@ 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. > > 'zap_empty_pte_table() may have cleared the PMD but not yet modified the mapped > folios' > > Is that accurate after the patch? > > You run tlb_flush_rmaps() before zap_empty_pte_table() now, and thus > folio_remove_rmap_ptes() ran for every folio by then. > > Maybe something like: > > "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... " Ack. Thanks. Looks good to me. >> * >> * Wait for concurrent zapping to complete by grabbing the PTL. >> */ >> diff --git a/mm/memory.c b/mm/memory.c >> index 6a8e7772b8d6..8f7ba427371b 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); >> + } >> + > > Maybe worth a comment here about ordering? Sure. Will add something like below: diff --git a/mm/memory.c b/mm/memory.c index 976cd03954e3..6a93b7fa48b2 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2052,6 +2052,9 @@ 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); >> /* >> * Fast path: try to hold the pmd lock and unmap the PTE page. >> * >> @@ -2046,15 +2055,6 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, >> */ >> 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 +2102,11 @@ 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); >> } > > I think David's getting at (correct me if I'm wrong) doing: > > } else if (details && details->single_folio && pmd_none(*pmd)) { > sync_with_folio_pmd_zap(tlb->mm, pmd); > } > > Here, rather than folding into the below? > > I think that's neater also. As I replied to David, keeping two pmd_none() checks cannot fix the race.