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[2003:cb:c710:8800:a73c:ec5b:c02c:5e0b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-35ef5fc0a1csm844203f8f.90.2024.06.06.01.01.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Jun 2024 01:01:44 -0700 (PDT) Message-ID: <758f7be7-c17e-46d1-879f-83340ec85749@redhat.com> Date: Thu, 6 Jun 2024 10:01:43 +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: 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> 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 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 and should try current mm/mm-unstable with MADV_FREE + mlock() on a PMD-mapped THP. > >> >> >> 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. > > I added the "!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD))" check > in this patch just to future-proof mlock for a PMD-mapped THP missing > the mlock, to prevent it from being picked up during page reclaim. > > But is this really required? It seems like nothing should really be broken > without this check. > > Perhaps, we should drop it from this patch until we fully understand the > reason for it. Could you get me some suggestions? We should drop it from this patch, agreed. We might need it ("!pvmw.pte") in patch #3, but I still have to understand if there really would be a problem. -- Cheers, David / dhildenb