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Fri, 06 Nov 2026 09:29:55 +0000 X-Mizu-Trace-ID: 0dede42dda7479ec X-Migadu-Flow: FLOW_OUT Message-ID: <09095de1-3916-43a9-a217-ae8564ba4d4c@linux.dev> Date: Fri, 6 Nov 2026 17:29:36 +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 Content-Language: en-US To: baolin.wang@linux.alibaba.com Cc: baohua@kernel.org, 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: <453e2c18-4e79-4db2-b565-1af881925f08@linux.alibaba.com> <20261106091510.67980-1-lance.yang@linux.dev> From: Lance Yang In-Reply-To: <20261106091510.67980-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: hunf6eb6kmseuop483emey44u5ghueaq X-Rspamd-Queue-Id: 3AF1140005 X-Rspam-User: X-Rspamd-Server: rspam08 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-HE-Tag: 1793957396-216778 X-HE-Meta: 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 vHd8each HS+dzYEQg+cIdWPwerjWjqcOd4qnTL9EEn/mKPZAkQIzDR4IvvZXdtPnfaJqjVPTqpt/rGOZoUYDAfpZLFHcSFtcHiHW3KWF8YRhfuecazml8j6zdUGngRgOwXjR2pB8pblUPAVd8V50wiGEgapRrio/Vo8xomaziVtZpW+WondahBmbTcSwKtM3MsmEWSXDO/VPv2T/moApxJyCSctWGRSncGQjiMI3Y4Wdj1D93ua9wsYkVlyOUJNOLz58dLXh6O6NFhBAiRsGtNBPu9UhXHCaTjAw+Zpw8qy3PyYrwu+2MLncWSt+2OuPhzYyyZZWPwXHcAUT9/PNatOe6zBi9ztc22Fd0xKReMhZ0 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 2026/11/6 17:15, Lance Yang wrote: > > On Fri, Nov 06, 2026 at 04:54:44PM +0800, Baolin Wang wrote: >> >> >> On 11/6/26 2:24 PM, Barry Song wrote: >>> On Thu, Nov 5, 2026 at 5:00 PM Baolin Wang >>> wrote: >>>> >>>> CC Barry, who added the TTU_SYNC in shrink_folio_list(). >>>> >>>> On 11/5/26 1:48 PM, Lance Yang wrote: >>>>> >>>>> >>>>> 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? >>>> >>>> Thanks Lance. But this matches the purpose of the TTU_SYNC flag, which >>>> is used to avoid racy checks with PVMW_SYNC. If no TTU_SYNC flag is set, >>>> it means the caller can tolerate some racy checks. >>>> >>>> For this case, I think we should set TTU_SYNC unconditionally in >>>> shrink_folio_list(). Barry, what do you think? >>> >>> Baolin, thanks for reaching out to me. I added that flag >>> mainly for large folios. >>> >>> I observed cases where PTEs for large folios are partially >>> cleared during certain transition states, for example, when >>> PTEs are cleared before being modified: >>> >>> pte = ptep_get_and_clear(); >>> pte modification; >>> set_pte_at(); >>> >>> In such cases, page_vma_mapped_walk() skips the empty >>> entries and starts from the next valid one. >>> >>> This may result in partially unmapping PTEs for large folios >>> and trigger unexpected folio splits. At the time, I was >>> particularly concerned about unnecessary folio splitting, >>> so my focus was mainly on large folios. >> >> Thanks for the information. >> >>> However, I also noticed that a similar race can occur with >>> small folios. In that case, the folio may simply remain >>> unreclaimed by the reclamation path. >>> >>> However, my main concern at the time was avoiding >>> unnecessary folio splits. >>> >>> Your new race also seems to amplify the same situation. >>> However, I don't think this problem necessarily depends on >>> the commit you mentioned. It has been around for a long time. >> >> Agree. >> >>> If the concern is reclaim failure, I agree that there is no >>> fundamental difference between large folios and small folios. >> >> OK. And I did a quick measurement to reclaim anon and file folios via >> the memory.reclaim interface. >> >> From test results, reclaiming 1G small anonymous folios (using zram as >> swap) shows no significant performance change. However, reclaiming 1G >> mapped clean small file folios does show an observable regression if the >> TTU_SYNC flag is set. >> >> Without TTU_SYNC for small folios: >> time echo 1G > /sys/fs/cgroup/test/memory.reclaim >> real 0m0.178s >> user 0m0.000s >> sys 0m0.178s >> >> With TTU_SYNC for small folios: >> real 0m0.315s >> user 0m0.000s >> sys 0m0.314s > > Nice numbers, cool! > >> So for the current patch, I'd prefer to keep things as is: only sync >> when TTU_SYNC is set. Without setting the TTU_SYNC flag, keep the >> original logic, meaning the caller can tolerate some racy checks. >> >> Any other thoughts? > > One thing I want to pin down first, for the order-0 folios, was that run > with TTU_SYNC going through the pmd lock, or did it already go the > smp_rmb() way? > > My guess is the first one, right? > > If so, please keep the fix as is and only sync when the caller asks for > it (at least, order-0 folios should not go the pmd lock way) :D In addition: For order-0 folios, we'd miss reclaim and reactivate a cold folio. That doesn't sound too bad, especially for such a rare race. so keeping the current behavior seems reasonable given the sync overhead ;) > Cheers, Lance