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Thu, 05 Nov 2026 05:48:54 +0000 X-Mizu-Trace-ID: 3f45584444935ba1 X-Migadu-Flow: FLOW_OUT Message-ID: <0ddaf336-7cb7-4e83-af2c-f60d23f7aafd@linux.dev> Date: Thu, 5 Nov 2026 13:48:42 +0800 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes To: baolin.wang@linux.alibaba.com Cc: akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, qi.zheng@linux.dev, pfalcato@suse.de, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261105045406.39871-1-lance.yang@linux.dev> Content-Language: en-US From: Lance Yang In-Reply-To: <20261105045406.39871-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Stat-Signature: ptx8hczcguqfkqcgoej17d71ipdmrj9m X-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Queue-Id: 798B6C0006 X-Rspam-User: X-HE-Tag: 1793857736-694432 X-HE-Meta: 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 DMnWsgXe 8ulmYxJWZvDOm+LwrrkRtfBV6CTNYVPFk07dic8U0ePgDyVT78f/l5GuLFLAWylOw9aqDZqp100q8iApwO42Eh8OOe7LVTz9Ow+2kgnaKt8Sta1WsOk/JvDuShQDLxC4Gfi/5+7Yr6Ua8kxn9xhnV0C8S7QCFaV7KyO5k1gP907cZkjkweZz1niZudpfLzJQyJBisHEaABzb6QuMqJtrA2Hs6xdiLiCgJ9L32dnAvgeI0rs06/P+RIyu6EZVyv1Xsg+5VDu27c6CX7AlvZ9SIOjPbqhvpIwo07PymEBPBqJ1to/DeSBLDPReibnFlJkbKc4nV Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 2026/11/5 12:54, Lance Yang wrote: > > On Thu, Nov 05, 2026 at 10:56:28AM +0800, Baolin Wang wrote: >> Similar to commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped >> folios"), the '!pmd_present()' check in page_vma_mapped_walk() can also race >> with concurrent page table reclamation via zap_empty_pte_table(). > > Good catch! > >> page_vma_mapped_walk() is used by many functions including try_to_unmap(), >> try_to_migrate(), folio_mkclean(), pfn_mkclean_range(), remove_migration_ptes(), >> page_mapped_in_vma(), and mapping_wrprotect_range(). Except for try_to_unmap() >> and try_to_migrate(), the other callers do not perform post-walk accounting >> checks, so the race has no observable impact on them. >> >> For migration, the race can cause unnecessary retries. For reclaim via >> try_to_unmap(), the race can cause failure to reclaim a folio that is in >> fact already unmapped (as shown below), which may lead to unexpected OOM >> or increased refaults. >> >> CPU 0 (reclaim) CPU 1 (MADV_DONTNEED) >> -------------------------------- -------------------------------- >> folio_lock(F) >> try_to_unmap(F, TTU_SYNC) >> zap_pte_range(full PMD range) >> spin_lock(ptl) >> clear PTEs >> zap_empty_pte_table() >> spin_trylock(pml) >> pmd_clear() >> spin_unlock(pml) >> >> page_vma_mapped_walk() >> observe !pmd_present(pmde) >> nr_pages < HPAGE_PMD_NR >> skip synchronization >> return false >> >> try_to_unmap() returns >> folio_mapped(F) == true >> nr_unmap_fail += folio_nr_pages(F) >> tlb_flush_mmu_tlbonly() >> tlb_flush_rmaps() >> remove F's rmaps >> spin_unlock(ptl) >> >> Commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped folios") >> has already moved the PMD clear operation (via zap_empty_pte_table()) to >> after tlb_flush_rmaps(). Similarly, page_vma_mapped_walk() also needs to >> use sync_with_folio_pmd_zap() to synchronize and avoid memory ordering >> issues on weakly ordered CPUs. >> >> Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)") >> Cc: stable@vger.kernel.org >> Suggested-by: David Hildenbrand (Arm) >> Signed-off-by: Baolin Wang >> --- >> mm/page_vma_mapped.c | 5 +---- >> 1 file changed, 1 insertion(+), 4 deletions(-) >> >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >> index e6b6170c2f4f..8797467b6f0c 100644 >> --- a/mm/page_vma_mapped.c >> +++ b/mm/page_vma_mapped.c >> @@ -293,10 +293,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >> spin_unlock(pvmw->ptl); >> pvmw->ptl = NULL; >> } else if (!pmd_present(pmde)) { >> - if ((pvmw->flags & PVMW_SYNC) && >> - thp_vma_suitable_order(vma, pvmw->address, >> - PMD_ORDER) && >> - (pvmw->nr_pages >= HPAGE_PMD_NR)) >> + if (pvmw->flags & PVMW_SYNC) > > Right. Whether the folio can be PMD-mapped doesn't tell us whether its > PTE table can be reclaimed, so those extra checks can skip synchronization > that the caller explicitly asked for ;) Ouch ... one thing I missed ... what about order-0 reclaim? shrink_folio_list() only sets TTU_SYNC for large folio, so can't we still hit the same race there? Cheers, Lance > LGTM, thanks! > > Reviewed-by: Lance Yang > >> sync_with_folio_pmd_zap(mm, pvmw->pmd); >> >> goto next_pmd; >> -- >> 2.47.3 >> >>