From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-119.freemail.mail.aliyun.com (out30-119.freemail.mail.aliyun.com [115.124.30.119]) (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 72F9E46C4BF; Fri, 9 Oct 2026 06:46:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.119 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791528394; cv=none; b=IZz13OAT6vjyx6R+qRxFUMDnA30MsbgecK4+g01TQU/yuvINsEWAK4xNHIoHBOj+tsQ3D1y9Ccr0aYcJ4SZPGQlLn7iUPLnT8uPEx+zIqIqkS3yETeUZ2uQZBOip7TXCaIqtNv5JJN9VWEaMWVrEzNSXuSSC6cdvCd7rzKdJMCs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791528394; c=relaxed/simple; bh=dlY3yFfM2QAMxT65i2r5NnPCMBMFXG5ZIXM8ycYAewY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Bdw/lnq0cfCI/AicQr1lZqJ/dKgqvOOQ7755icZKur5w6bBV+B1jLK1guFLQg1z9E+nENh62z1Whdz9baIDs6+6nmwWlbjozUkLwxDtEmyXfwzPaFjM1qsICI5EGgMZKBUwz4RZfpM3HgMP3YuUgSNILfx3x74cAVKrtYdV6/Xg= 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=AKKN8lST; arc=none smtp.client-ip=115.124.30.119 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="AKKN8lST" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1791528381; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=LoVx2R5PChUvqrXRet/NFBqBIchEhynSZ3ARypBye7Q=; b=AKKN8lSTU2LpoNaJKW3gPwn6QEjA2sHAc78IlJRong9dhBDrZ3/5Dg4Kl14eac+YYHNFIImkNm1lNQNe9eLFVUV3E3ESTOJlabX+MftnCuZazLLu1AdqAyJ0MES1wG/M1KSW6W9KmJ1FrKW+AoxEuAr0eFTTxYkvH62NQWYXmAA= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R331e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033032089153;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=11;SR=0;TI=SMTPD_---0XCSZjHs_1791528378; Received: from 30.74.144.136(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XCSZjHs_1791528378 cluster:ay36) by smtp.aliyun-inc.com; Fri, 09 Oct 2026 14:46:19 +0800 Message-ID: Date: Fri, 9 Oct 2026 14:46:18 +0800 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [BUG] shmem: FALLOC_FL_PUNCH_HOLE vs fault-around race corrupts page cache / rss counters To: Jan Kara , Andrew Morton Cc: Ayush Ranjan , Pedro Falcato , Hugh Dickins , Matthew Wilcox , David Hildenbrand , Gregory Price , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260924061708.1645968-1-ayushr@modal.com> <20260925053027.1998394-1-ayushr@modal.com> <20260925065013.3682431-1-ayushr@modal.com> <20261003033107.1488699-1-ayushr@modal.com> <20261004222831.bdd648a2b406f02747dd9e40@linux-foundation.org> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/8/26 5:24 PM, Jan Kara wrote: > On Sun 04-10-26 22:28:31, Andrew Morton wrote: >> On Sat, 3 Oct 2026 03:31:03 +0000 Ayush Ranjan wrote: >>> Gentle ping on this. The reproducer in my previous mail [1] triggers >>> "Bad page cache ... still mapped when deleted" on 6.18.46 within a >>> couple of minutes on a 128-CPU bare-metal box, with no fork() and no >>> gVisor involved. >>> >>> We continue to hit this in production at low frequency, so I am happy >>> to test patches or collect more data if that would help. >>> >> >> fwiw I made gpt and gemini argue about this for a while and ended up >> with the below. > > Worth a try I guess. Let's see :) > >> From: Andrew Morton >> Subject: mm: shmem: serialize fault-around against hole punching >> Date: Sun Oct 4 09:05:31 PM PDT 2026 >> >> shmem uses filemap_map_pages() for fault-around. Unlike shmem_fault(), >> filemap_map_pages() does not participate in shmem's fallocate exclusion >> protocol. >> >> During a partial hole punch of a large shmem folio, the folio can be split >> and the resulting smaller folios can remain temporarily visible in the page >> cache while shmem_undo_range() restarts its walk. Fault-around can then map >> one of those folios again before the hole-punch path removes it. >> >> shmem_undo_range() may subsequently delete that folio from the page cache >> despite the new userspace mapping. This can trigger "still mapped when >> deleted" warnings and leave stale mappings or inconsistent RSS/page-table >> accounting behind. > > So this part of explanation is either incomplete or wrong in my opinion. > Yes, shmem_undo_range() calls truncate_inode_partial_folio() which can > split a large folio. Yes, filemap_map_pages() can map those pages back into > page tables. But how "shmem_undo_range() may subsequently delete that folio > from the page cache despite the new userspace mapping" happens is unclear > to me. After splitting a folio, shmem_undo_range() will restart and find the > newly split (and mapped) folios and calls truncate_inode_folio() to get rid > of them. Now truncate_inode_folio() calls truncate_cleanup_folio() which > does: > > if (folio_mapped(folio)) > unmap_mapping_folio(folio); > > so the mapping is reliably removed under folio lock. Can you perhaps push > your agents further to explain in more detail how this "still mapped when > deleted" happens in their opinion? Good point and I think you are right. Yesterday I quickly reproduced the issue with Ayush's reproducer, 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. I also quickly tried Andrew's patch, and the issue no longer reproduces, so I initially thought that was the root cause. But after your reminder, I now believe Andrew's patch merely workaround the issue rather than fixing the actual root cause. Today I'm going to re-analyze the race with the reproducer (thanks Ayush). After analysis, I believe the race exists between truncation and MADV_DONTNEED, and shmem's fault_around() 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() but still observe the folio's mapcount as non-zero. A possible race scenario is as follows: CPU 0 CPU 1 madvise_dontneed_single_vma shmem_fallocate ...... ...... zap_pte_range truncate_inode_folio zap_empty_pte_table(pmd clear) unmap_mapping_folio ...... zap_pmd_range(saw pmd none) filemap_remove_folio BUG_ON(folio_mapped) tlb_flush_rmaps Based on the above race analysis, I made the following fix that uses the PMD lock synchronously to prevent this race, and the issue no longer reproduces. I will clean it up and send out a formal patch. diff --git a/mm/memory.c b/mm/memory.c index 6a8e7772b8d6..2039ada99b64 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. * @@ -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); /* @@ -2103,7 +2103,6 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb, } /* 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); } [ 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) [ 189.172013] raw: 017fffef0002022d fffffdffccec85c8 fffffdffc6b8e508 ffff0000cd2ab6c0 [ 189.172015] raw: 0000000000003880 0000000000000000 0000000300000000 ffff0000cc073d40 [ 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: G E 7.3.0-rc4+ #277 PREEMPT(lazy) [ 189.178320] Tainted: [E]=UNSIGNED_MODULE ....... [ 189.179339] pc : filemap_unaccount_folio+0xf0/0x1e8 [ 189.179619] lr : filemap_unaccount_folio+0xf0/0x1e8 ....... [ 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 [ 189.185764] __arm64_sys_fallocate+0x50/0xa0 [ 189.185999] invoke_syscall+0x58/0x118 [ 189.186210] el0_svc_common.constprop.0+0xbc/0xe8 [ 189.186470] do_el0_svc+0x20/0x30 [ 189.186653] el0_svc+0x3c/0x198 [ 189.186843] el0t_64_sync_handler+0x98/0xe0 [ 189.187078] el0t_64_sync+0x184/0x188 [ 189.187622] SMP: stopping secondary CPUs [ 189.200152] Starting crashdump kernel... [ 189.200388] Bye!