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Received: from 30.74.144.136(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XCT-5GK_1791537623 cluster:ay36) by smtp.aliyun-inc.com; Fri, 09 Oct 2026 17:20:24 +0800 Message-ID: <5ebaba53-b2de-490e-a14c-c49bbb4337ad@linux.alibaba.com> Date: Fri, 9 Oct 2026 17:20:23 +0800 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: 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> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: o5ugb6w9bmxz9pp3oc3ukrxjqxu41w7d X-Rspam-User: X-Rspamd-Queue-Id: E408820008 X-Rspamd-Server: rspam08 X-HE-Tag: 1791537628-515254 X-HE-Meta: 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 It9xomke D937o1WK72WdFtq9UKpFz6TJdpS+AyYQFs6ZnB783P4OMcQ02MWlNaczLs+VCnEFeQpehU79Sr0GiS+bbL0CBt7BMTK81cFBBTVbVEWgOCAwn+kO2+5QPHM4C9YVZj7/TKQ5kkquTnn+4UHmVHCLjHOMZo3364kv111mtuuAYlRU6bmlQv42PRnuEsAk5dh0FSCYphuQC1wUOvlInqQU4RQ0FLYduRHItlw6FNDaK2y8LLsAgZ0FH8yPn4/1GPeBDVVtbNZfQGE1OMDEVA04hoQnpElw8nvnZ8x4wlXr33cfVpCdZqVBrcZqM3cJnIO5cJhjRweN2jD1+NJSirqRAj/SfPaPVOu0S+bWN6+W5+B4aHJi/Y4w5L0oDt3y4xLCwPW0gr5fyHs5UE6TbirDB2tOxRVg5KZ7XcWXIWa5PWQ4MpDQkeNDUQOUBUKrfc/AaU8UctWQLZzjHUJ8AfhD4bXPeCNN2iV0Mpfff Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. /* >> @@ -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) && > > Why? Ah, sorry, this change can be removed.