From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.223.131]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DB5FB53F69B for ; Wed, 14 Oct 2026 14:05:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=195.135.223.131 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791986721; cv=none; b=gj7fYSyUKzbSUTcKndTadRufar5bfNZkOf7TIXLjNvmAWr5ewHBWjQgEXwr/HanuLfQXZ50GAJ0+Flz4CcMigo25aoEDnTduei162aGEpi95WRsJ+5ZkVSovkfHD7BgjYfKlThAQTsHazYsy8A4J10SMFbibGw6wGP8oKd5aiTc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791986721; c=relaxed/simple; bh=BcS1DfKG3o26wsOhDMqs7ZNTVdrEU4sz2ASwsqj0Eoo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ElkmuC7hTRw3o013jYH3E+iocEW4yqEStpWSI7bg9i7bUCTLLqipgZvXD/XU+uDj/WMrmwZML721WDMIPnHWY/POOLaAXp1T1Nmhz3kqpuugTK/99Pfm0uTW4tml35ai0+PsAmzo2jA79/fsLLybBUKExzkeUNe846ev4R21OxM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=suse.de; spf=pass smtp.mailfrom=suse.de; dkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de header.b=0XvkIx6Z; dkim=permerror (0-bit key) header.d=suse.de header.i=@suse.de header.b=P0oixPkI; dkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de header.b=0XvkIx6Z; dkim=permerror (0-bit key) header.d=suse.de header.i=@suse.de header.b=P0oixPkI; arc=none smtp.client-ip=195.135.223.131 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=suse.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=suse.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de header.b="0XvkIx6Z"; dkim=permerror (0-bit key) header.d=suse.de header.i=@suse.de header.b="P0oixPkI"; dkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de header.b="0XvkIx6Z"; dkim=permerror (0-bit key) header.d=suse.de header.i=@suse.de header.b="P0oixPkI" Received: from imap1.dmz-prg2.suse.org (unknown [10.150.64.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id B7CF31FBB7; Wed, 14 Oct 2026 14:05:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1791986714; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/jzBRIumIkRQfzVUhv0b17LsLJAndLvzauLlUMqudqo=; b=0XvkIx6ZsNBk27nnMlmBbhpsARlPLSs0NpDQW1OdCoZlEvkpBwC54O5c8HO89zXv4r0Wq7 6pv6mvgjrOXNpGCJtc+PwfkpvO88lEU99z82bAupoxafwV9RXr7eLERl1BJwIbon4nnVUn tbSeNEue0lx7erI1P59EhziUN1zsSBw= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1791986714; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/jzBRIumIkRQfzVUhv0b17LsLJAndLvzauLlUMqudqo=; b=P0oixPkIAon4rHjZ8LShiwbrjZxTnqzwWCZVGCVSGljNzBfFZZTySaPpQrPhuFOyybRf0h WKgb27CyPPea8VAA== Authentication-Results: smtp-out2.suse.de; none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1791986714; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/jzBRIumIkRQfzVUhv0b17LsLJAndLvzauLlUMqudqo=; b=0XvkIx6ZsNBk27nnMlmBbhpsARlPLSs0NpDQW1OdCoZlEvkpBwC54O5c8HO89zXv4r0Wq7 6pv6mvgjrOXNpGCJtc+PwfkpvO88lEU99z82bAupoxafwV9RXr7eLERl1BJwIbon4nnVUn tbSeNEue0lx7erI1P59EhziUN1zsSBw= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1791986714; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/jzBRIumIkRQfzVUhv0b17LsLJAndLvzauLlUMqudqo=; b=P0oixPkIAon4rHjZ8LShiwbrjZxTnqzwWCZVGCVSGljNzBfFZZTySaPpQrPhuFOyybRf0h WKgb27CyPPea8VAA== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 66BEA133F1; Wed, 14 Oct 2026 14:05:12 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id au8sMReMz2qmZAAAD6G6ig (envelope-from ); Wed, 14 Oct 2026 14:05:12 +0000 Date: Wed, 14 Oct 2026 15:05:10 +0100 From: Pedro Falcato To: "David Hildenbrand (Arm)" Cc: Baolin Wang , akpm@linux-foundation.org, 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, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm: memory: fix truncation removing mapped folios Message-ID: References: <542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com> <8245424a-f068-4be5-9e58-713cbff1f8ae@kernel.org> <405811f5-ee36-4b3c-80d0-7dc6827ab5d4@linux.alibaba.com> <6660d3bb-3485-4c34-8734-3ea99df920f1@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6660d3bb-3485-4c34-8734-3ea99df920f1@kernel.org> X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-0.999]; MIME_GOOD(-0.10)[text/plain]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[14]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo] X-Spam-Score: -4.30 X-Spam-Level: X-Spam-Flag: NO On Wed, Oct 14, 2026 at 03:25:33PM +0200, David Hildenbrand (Arm) wrote: > 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(). Hmm, could we viably replace sync_with_folio_pmd_zap() with the memory barriers instead? Seems lighter-weight and functionally equivalent? -- Pedro