From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-99.freemail.mail.aliyun.com (out30-99.freemail.mail.aliyun.com [115.124.30.99]) (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 4F9F02737F9; Fri, 9 Oct 2026 10:14:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.99 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791540850; cv=none; b=nVKzkuVE/mRBH3SfLJIV2vFBoUv04FPmKNW58c2C2QS2x/4Axu+XG46Tl2eiK1zqK8x6XnJCV0hJ9p12GNJ5SlteH7GXYM54PiAtTlEbrLYLQLzRoN2hbqMnw7yLhl9MK7Wob174Q8tXvyoXQ7ypfSOUVtAqU9bDHpg3FL5/Iro= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791540850; c=relaxed/simple; bh=/KxgX8F4YwpJXd5C12Xkck3LwZuZJq41LVY+9W+7P2w=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=d7Q0nBc3BhQNDad/8CwKvmdh1N3ufGE0Ik9SVy3zrJjJqNiB8Bm62qQilFjjTKrlO3mwTKp//wbF3ySaRf3/fttItMV6nydI5D5DB1HtUj3pd1LynupvWTbWiDCSWgoesBKx/pAvSPj8bV9h+RrTUM2Y1jLkFOlCpotF8aPByeE= 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=byaqqJyX; arc=none smtp.client-ip=115.124.30.99 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="byaqqJyX" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1791540845; h=Message-ID:Date:MIME-Version:Subject:From:To:Content-Type; bh=Mu/ZQABM8qhqcMl44qsHwngC77x8JwfLP3WIFCwDal8=; b=byaqqJyX2qUr863wX7TDc5QUNiBw1GRY3Yw26kLEldx6Js/Srvq+nnJPG/qKTLXGfEyYur+NN3wkD+3C1FycmjhRx8sApLt4PXYXPLHEO7DmedfQoiRqW3JHP/BEPTUBuctrBjdo4tE+bXoj7d3n5kIABXTtgTsL5LYZLxhN0kg= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R381e4;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_---0XCT5rY._1791540843; Received: from 30.74.144.136(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XCT5rY._1791540843 cluster:ay36) by smtp.aliyun-inc.com; Fri, 09 Oct 2026 18:14:04 +0800 Message-ID: <639389d5-5048-4145-bcbf-c4b2d57e86e8@linux.alibaba.com> Date: Fri, 9 Oct 2026 18:14:03 +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 From: Baolin Wang To: Pedro Falcato Cc: Jan Kara , Andrew Morton , Ayush Ranjan , 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> <5ebaba53-b2de-490e-a14c-c49bbb4337ad@linux.alibaba.com> In-Reply-To: <5ebaba53-b2de-490e-a14c-c49bbb4337ad@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10/9/26 5:20 PM, Baolin Wang wrote: > > > On 10/9/26 4:41 PM, Pedro Falcato wrote: >> On Fri, Oct 09, 2026 at 02:46:18PM +0800, Baolin Wang wrote: >>> >>> >>> 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 >> >> Thanks for the investigation! >> >> So, I think I understand the problem (rmap walks race PTE zapping, >> which no >> longer serializes on the PTE lock), but I don't understand your >> solution at >> all. >>> >>> 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); >> >> Namely, why moving this hunk of code up there makes any difference. >> I suppose it's subtly changing memory order, but it's not immediate >> in any way why this is correct. >> >> What you really need (I think) is a happens-before relationship between >> the rmap changes and the PMD zapping. Something like: >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 330cde31bf8b..17cdd48fff27 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -2108,6 +2108,13 @@ static inline unsigned long >> zap_pmd_range(struct mmu_gather *tlb, >>                          sync_with_folio_pmd_zap(tlb->mm, pmd); >>                  } >>                  if (pmd_none(*pmd)) { >> +                       /* >> +                        * Pairs with tlb_flush_rmaps() in PTE zapping. >> +                        * Possibly skipping a page table (due to PTE >> zapping) >> +                        * needs to enforce ordering between the rmap >> changes >> +                        * and the PMD getting cleared. >> +                        */ >> +                       smp_rmb(); >>                          addr = next; >>                          continue; >>                  } >> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c >> index 9f353f0e2ef4..47b25c42bf77 100644 >> --- a/mm/mmu_gather.c >> +++ b/mm/mmu_gather.c >> @@ -89,6 +89,10 @@ void tlb_flush_rmaps(struct mmu_gather *tlb, struct >> vm_area_struct *vma) >>          tlb_flush_rmap_batch(&tlb->local, vma); >>          if (tlb->active != &tlb->local) >>                  tlb_flush_rmap_batch(tlb->active, vma); >> +       /* >> +        * rmap changes need to be observed before e.g PTEs get zapped. >> +        */ >> +       smp_wmb(); >>          tlb->delayed_rmap = 0; >>   } >>   #endif >> >> On top of your diff. Where the smp_wmb() is perhaps not required: I >> suppose TLB flushing >> works as a sort of memory barrier itself on most/all architectures. >> Either way, we have to document and fix expectations around this code. > > Agree, I don't think we need smp_wmb() here. Moreover, > zap_empty_pte_table() will call the pmd lock/unlock, which already > indicates a memory barrier. After more thinking, I think we still need an smp_wmb(). Please review my new fix patch. Thanks. https://lore.kernel.org/all/0d2a1809fa1cfc22ad3df26ec28b2d30e5cdf3bd.1791540483.git.baolin.wang@linux.alibaba.com/