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[2003:cb:c706:3100:19a8:d898:8e69:6aff]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4215811d981sm24111095e9.25.2024.06.05.08.02.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 05 Jun 2024 08:02:54 -0700 (PDT) Message-ID: <7f2ab112-5916-422c-b29f-343cc0d6d754@redhat.com> Date: Wed, 5 Jun 2024 17:02:52 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop To: Lance Yang Cc: Yin Fengwei , 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, 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> <8580a462-eadc-4fa5-b01a-c0b8c3ae644d@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 05.06.24 16:57, Lance Yang wrote: > On Wed, Jun 5, 2024 at 10:39 PM David Hildenbrand wrote: >> >> On 05.06.24 16:28, 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. >>> >>> 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)". >>> >>> After this patch, we either >>> >>> 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)". >>> >>> 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? >>> >>> > > Thanks a lot for clarifying! > >>> So if the check is required in this patch, we really have to understand >>> why. If not, we should better drop it from this patch. >>> >>> At least my opinion, still struggling to understand why it would be >>> required (I have 0 knowledge about mlock interaction with large folios :) ). >>> >> >> Looking at that series, in folio_references_one(), we do >> >> if (!folio_test_large(folio) || !pvmw.pte) { >> /* Restore the mlock which got missed */ >> mlock_vma_folio(folio, vma); >> page_vma_mapped_walk_done(&pvmw); >> pra->vm_flags |= VM_LOCKED; >> return false; /* To break the loop */ >> } >> >> I wonder if we want that here as well now: in case of lazyfree we >> would not back off, right? >> >> But I'm not sure if lazyfree in mlocked areas are even possible. >> >> Adding the "!pvmw.pte" would be much clearer to me than the flag check. > > Hmm... How about we drop it from this patch for now, and add it back if needed > in the future? If we can rule out that MADV_FREE + mlock() keeps working as expected in the PMD-mapped case, we're good. Can we rule that out? (especially for MADV_FREE followed by mlock()) -- Cheers, David / dhildenb