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[2003:cb:c710:8800:a73c:ec5b:c02c:5e0b]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4216055101dsm561185e9.21.2024.06.06.01.06.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Jun 2024 01:06:44 -0700 (PDT) Message-ID: <5a728148-ed93-4d68-a86f-9be3612dedbb@redhat.com> Date: Thu, 6 Jun 2024 10:06:42 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop From: David Hildenbrand To: Lance Yang Cc: akpm@linux-foundation.org, willy@infradead.org, sj@kernel.org, baolin.wang@linux.alibaba.com, maskray@google.com, ziy@nvidia.com, ryan.roberts@arm.com, 21cnbao@gmail.com, mhocko@suse.com, fengwei.yin@intel.com, zokeefe@google.com, shy828301@gmail.com, xiehuan09@gmail.com, libang.li@antgroup.com, wangkefeng.wang@huawei.com, songmuchun@bytedance.com, peterx@redhat.com, minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240521040244.48760-1-ioworker0@gmail.com> <20240521040244.48760-3-ioworker0@gmail.com> <758f7be7-c17e-46d1-879f-83340ec85749@redhat.com> Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: 7be9bsbz4tzcgfnmzxrotdbct6bbbin8 X-Rspamd-Queue-Id: D4A5F1C0023 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1717661208-499495 X-HE-Meta: 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 oGja8Rvc 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 06.06.24 10:01, David Hildenbrand wrote: > On 06.06.24 05:55, Lance Yang wrote: >> On Wed, Jun 5, 2024 at 10:28 PM David Hildenbrand wrote: >>> >>> On 05.06.24 16:20, Lance Yang wrote: >>>> Hi David, >>>> >>>> On Wed, Jun 5, 2024 at 8:46 PM David Hildenbrand wrote: >>>>> >>>>> On 21.05.24 06:02, Lance Yang wrote: >>>>>> In preparation for supporting try_to_unmap_one() to unmap PMD-mapped >>>>>> folios, start the pagewalk first, then call split_huge_pmd_address() to >>>>>> split the folio. >>>>>> >>>>>> Since TTU_SPLIT_HUGE_PMD will no longer perform immediately, we might >>>>>> encounter a PMD-mapped THP missing the mlock in the VM_LOCKED range during >>>>>> the page walk. It’s probably necessary to mlock this THP to prevent it from >>>>>> being picked up during page reclaim. >>>>>> >>>>>> Suggested-by: David Hildenbrand >>>>>> Suggested-by: Baolin Wang >>>>>> Signed-off-by: Lance Yang >>>>>> --- >>>>> >>>>> [...] again, sorry for the late review. >>>> >>>> No worries at all, thanks for taking time to review! >>>> >>>>> >>>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>>> index ddffa30c79fb..08a93347f283 100644 >>>>>> --- a/mm/rmap.c >>>>>> +++ b/mm/rmap.c >>>>>> @@ -1640,9 +1640,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>>>>> if (flags & TTU_SYNC) >>>>>> pvmw.flags = PVMW_SYNC; >>>>>> >>>>>> - if (flags & TTU_SPLIT_HUGE_PMD) >>>>>> - split_huge_pmd_address(vma, address, false, folio); >>>>>> - >>>>>> /* >>>>>> * For THP, we have to assume the worse case ie pmd for invalidation. >>>>>> * For hugetlb, it could be much worse if we need to do pud >>>>>> @@ -1668,20 +1665,35 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>>>>> mmu_notifier_invalidate_range_start(&range); >>>>>> >>>>>> while (page_vma_mapped_walk(&pvmw)) { >>>>>> - /* Unexpected PMD-mapped THP? */ >>>>>> - VM_BUG_ON_FOLIO(!pvmw.pte, folio); >>>>>> - >>>>>> /* >>>>>> * If the folio is in an mlock()d vma, we must not swap it out. >>>>>> */ >>>>>> if (!(flags & TTU_IGNORE_MLOCK) && >>>>>> (vma->vm_flags & VM_LOCKED)) { >>>>>> /* Restore the mlock which got missed */ >>>>>> - if (!folio_test_large(folio)) >>>>>> + if (!folio_test_large(folio) || >>>>>> + (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD))) >>>>>> mlock_vma_folio(folio, vma); >>>>> >>>>> Can you elaborate why you think this would be required? If we would have >>>>> performed the split_huge_pmd_address() beforehand, we would still be >>>>> left with a large folio, no? >>>> >>>> Yep, there would still be a large folio, but it wouldn't be PMD-mapped. >>>> >>>> After Weifeng's series[1], the kernel supports mlock for PTE-mapped large >>>> folio, but there are a few scenarios where we don't mlock a large folio, such >>>> as when it crosses a VM_LOCKed VMA boundary. >>>> >>>> - if (!folio_test_large(folio)) >>>> + if (!folio_test_large(folio) || >>>> + (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD))) >>>> >>>> And this check is just future-proofing and likely unnecessary. If encountering a >>>> PMD-mapped THP missing the mlock for some reason, we can mlock this >>>> THP to prevent it from being picked up during page reclaim, since it is fully >>>> mapped and doesn't cross the VMA boundary, IIUC. >>>> >>>> What do you think? >>>> I would appreciate any suggestions regarding this check ;) >>> >>> Reading this patch only, I wonder if this change makes sense in the >>> context here. >> >> Allow me to try explaining it again ;) >> >>> >>> Before this patch, we would have PTE-mapped the PMD-mapped THP before >>> reaching this call and skipped it due to "!folio_test_large(folio)". >> >> Yes, there is only a PTE-mapped THP when doing the "!folio_test_large(folio)" >> check, as we will first conditionally split the PMD via >> split_huge_pmd_address(). >> >>> >>> After this patch, we either >> >> Things will change. We'll first do the "!folio_test_large(folio)" check, then >> conditionally split the PMD via split_huge_pmd_address(). >> >>> >>> a) PTE-remap the THP after this check, but retry and end-up here again, >>> whereby we would skip it due to "!folio_test_large(folio)". >> >> Hmm... >> >> IIUC, we will skip it after this check, stop the page walk, and not >> PTE-remap the THP. >> >>> >>> b) Discard the PMD-mapped THP due to lazyfree directly. Can that >>> co-exist with mlock and what would be the problem here with mlock? >> >> Before discarding a PMD-mapped THP as a whole, as patch #3 did, >> we also perform the "!folio_test_large(folio)" check. If the THP coexists >> with mlock, we will skip it, stop the page walk, and not discard it. IIUC. > > But "!folio_test_large(folio)" would *skip* the THP and not consider it > regarding mlock. > > I'm probably missing something I'm stupid, I missed that we still do the "goto walk_done_err;", only that we don't do the mlock_vma_folio(folio, vma); Yes, let's drop it for now! :) -- Cheers, David / dhildenb