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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=h7s+2xSM; 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 B9C02606CF89 for ; Mon, 26 Oct 2026 10:00:36 +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=h7s+2xSM Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1DBCA1F01201; Mon, 26 Oct 2026 10:00:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793008836; bh=jShkiWdLlDgPXb5QV1zTppDSdv7OslzLO75z7LLDk5Y=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=h7s+2xSMry6qNTwi670I8Q9YEnaCNBKYZ1F/zhDMcnmQdWEbBZcqbufvJNZKkESx8 iaorYtQYdiKFXKURE68yn4sCh5n+Z+ZtOUcT7KA8CTYcMrNqFFs/8ogUExIX5y61Mu rzDT8gYjxbH0WbElBxHJmYiOQxLH1LDtXjQ5wUXLvyzpfvzONUHASqog/CWIW9JTIG E5lS52Xl0PyhGhNnTrOaFwDb1GKLVR+vKzjUXAv+ONQzIU4aRT+eoA9xfxtBWndYPn 9YRhYhL+9AqfHFx4RJWemfgOtWFj/zay+zB4XLbVtjUwS8YiosIXq6ZVbvqm4hka6L 2CDxoIKueZF0A== Message-ID: Date: Mon, 26 Oct 2026 11:00:29 +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 v4] 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: <4cc521a118564b4145edf0324066ab532fb41181.1792979870.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 10/26/26 03:05, 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 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 | 28 +++++++++++++++------------- > 2 files changed, 21 insertions(+), 17 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..6a93b7fa48b2 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 +2105,11 @@ 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); > } > + > if (pmd_none(*pmd)) { > + if (details && details->single_folio) > + sync_with_folio_pmd_zap(tlb->mm, pmd); > addr = next; > continue; > Sorry if I sound annoyed, but why don't you keep the } else if (pmd_none(*pmd)) { above as I suggested? Because we fancy unnecessary PMD table lock cycles? -- Cheers, David