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Fri, 9 Oct 2026 08:41:22 +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 C4YwMLKoyGpkJgAAD6G6ig (envelope-from ); Fri, 09 Oct 2026 08:41:22 +0000 Date: Fri, 9 Oct 2026 09:41:21 +0100 From: Pedro Falcato To: Baolin Wang 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 Subject: Re: [BUG] shmem: FALLOC_FL_PUNCH_HOLE vs fault-around race corrupts page cache / rss counters Message-ID: 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Pre-Result: action=no action; module=Unknown lua; unknown reason X-Rspamd-Pre-Result: action=no action; module=Unknown lua; unknown reason X-Rspamd-Queue-Id: 6428DC0002 X-Rspam-User: X-Rspamd-Server: rspam12 X-Stat-Signature: r9ryccqwc7uhxk5uhumpdcfobmjq6oag X-HE-Tag: 1791535285-64786 X-HE-Meta: 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 13VVTIsc I/3084BA3Vg+fuU4V4JBVeWQ1a5ItPItli8OMu3jVB9TTGWkVR5MGxvYM1tFfUk2DngH6y6gu7etKNNc8nsIqkROdF8OeaT+BVFQyayYL7gja/IhMOpNHpTaFFdcxqLjzs0PyHcpW5ZKClGLcT77r4dMMrg7gmvfLVcJyNsHrzyobtdIQMIzhWSD5VY3kYIy+9+wEVHba2/aw509sMv1dN5LE30hT60lCN/9Q3ZL2bNJUOpBeUfls5YuWGJQjkCXoPn4uBdBiWPEiz7cA71wYr71j6OGUEg2pOY3zb7RufU5T0e4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. > > /* > @@ -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? -- Pedro