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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id r12-20020adff10c000000b0033de2f2a88dsm20096450wro.103.2024.03.07.02.54.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 02:54:23 -0800 (PST) Message-ID: <8f84c7d6-982a-4933-a7a7-3f640df64991@redhat.com> Date: Thu, 7 Mar 2024 11:54:22 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free To: Ryan Roberts , Barry Song <21cnbao@gmail.com> Cc: Lance Yang , Vishal Moola , akpm@linux-foundation.org, zokeefe@google.com, shy828301@gmail.com, mhocko@suse.com, fengwei.yin@intel.com, xiehuan09@gmail.com, wangkefeng.wang@huawei.com, songmuchun@bytedance.com, peterx@redhat.com, minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240307061425.21013-1-ioworker0@gmail.com> <03458c20-5544-411b-9b8d-b4600a9b802f@arm.com> <501c9f77-1459-467a-8619-78e86b46d300@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: 9icsx917x96w4y5xijmqkidth3n1yjnc X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 5D54F2000C X-Rspam-User: X-HE-Tag: 1709808869-654587 X-HE-Meta: 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 U+EsRpkr 7jzGhr3moZoRwYj0szGaFRI3Ifsp+Se1qtZ8rK/ed1tlJzgGzV4UvUAJNwMNpWdIq0KJubEW8wKEQOUrh3oioAC44eLji4366toIjIeRfhFZE/gUSTOjNUSNzAq/8u4BQ9p37x6E40qPB3D6n9u5WiXqIYMb35/aGHOl8B2UYLCsE4CSmGOJ0d506RGJ9soAHv6Rvt/FIJrRyIlc2Vq1UUVVsPoUhRhlgqSxJ82U2hP4P6EgM+9noHN2M8vKvtPIzjSNut2WIJYBrNptlix2OpufAHI+u265fV7+r 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 07.03.24 11:50, Ryan Roberts wrote: > On 07/03/2024 09:33, Barry Song wrote: >> On Thu, Mar 7, 2024 at 10:07 PM Ryan Roberts wrote: >>> >>> On 07/03/2024 08:10, Barry Song wrote: >>>> On Thu, Mar 7, 2024 at 9:00 PM Lance Yang wrote: >>>>> >>>>> Hey Barry, >>>>> >>>>> Thanks for taking time to review! >>>>> >>>>> On Thu, Mar 7, 2024 at 3:00 PM Barry Song <21cnbao@gmail.com> wrote: >>>>>> >>>>>> On Thu, Mar 7, 2024 at 7:15 PM Lance Yang wrote: >>>>>>> >>>>> [...] >>>>>>> +static inline bool can_mark_large_folio_lazyfree(unsigned long addr, >>>>>>> + struct folio *folio, pte_t *start_pte) >>>>>>> +{ >>>>>>> + int nr_pages = folio_nr_pages(folio); >>>>>>> + fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >>>>>>> + >>>>>>> + for (int i = 0; i < nr_pages; i++) >>>>>>> + if (page_mapcount(folio_page(folio, i)) != 1) >>>>>>> + return false; >>>>>> >>>>>> we have moved to folio_estimated_sharers though it is not precise, so >>>>>> we don't do >>>>>> this check with lots of loops and depending on the subpage's mapcount. >>>>> >>>>> If we don't check the subpage’s mapcount, and there is a cow folio associated >>>>> with this folio and the cow folio has smaller size than this folio, >>>>> should we still >>>>> mark this folio as lazyfree? >>>> >>>> I agree, this is true. However, we've somehow accepted the fact that >>>> folio_likely_mapped_shared >>>> can result in false negatives or false positives to balance the >>>> overhead. So I really don't know :-) >>>> >>>> Maybe David and Vishal can give some comments here. >>>> >>>>> >>>>>> BTW, do we need to rebase our work against David's changes[1]? >>>>>> [1] https://lore.kernel.org/linux-mm/20240227201548.857831-1-david@redhat.com/ >>>>> >>>>> Yes, we should rebase our work against David’s changes. >>>>> >>>>>> >>>>>>> + >>>>>>> + return nr_pages == folio_pte_batch(folio, addr, start_pte, >>>>>>> + ptep_get(start_pte), nr_pages, flags, NULL); >>>>>>> +} >>>>>>> + >>>>>>> static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, >>>>>>> unsigned long end, struct mm_walk *walk) >>>>>>> >>>>>>> @@ -676,11 +690,45 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, >>>>>>> */ >>>>>>> if (folio_test_large(folio)) { >>>>>>> int err; >>>>>>> + unsigned long next_addr, align; >>>>>>> >>>>>>> - if (folio_estimated_sharers(folio) != 1) >>>>>>> - break; >>>>>>> - if (!folio_trylock(folio)) >>>>>>> - break; >>>>>>> + if (folio_estimated_sharers(folio) != 1 || >>>>>>> + !folio_trylock(folio)) >>>>>>> + goto skip_large_folio; >>>>>> >>>>>> >>>>>> I don't think we can skip all the PTEs for nr_pages, as some of them might be >>>>>> pointing to other folios. >>>>>> >>>>>> for example, for a large folio with 16PTEs, you do MADV_DONTNEED(15-16), >>>>>> and write the memory of PTE15 and PTE16, you get page faults, thus PTE15 >>>>>> and PTE16 will point to two different small folios. We can only skip when we >>>>>> are sure nr_pages == folio_pte_batch() is sure. >>>>> >>>>> Agreed. Thanks for pointing that out. >>>>> >>>>>> >>>>>>> + >>>>>>> + align = folio_nr_pages(folio) * PAGE_SIZE; >>>>>>> + next_addr = ALIGN_DOWN(addr + align, align); >>>>>>> + >>>>>>> + /* >>>>>>> + * If we mark only the subpages as lazyfree, or >>>>>>> + * cannot mark the entire large folio as lazyfree, >>>>>>> + * then just split it. >>>>>>> + */ >>>>>>> + if (next_addr > end || next_addr - addr != align || >>>>>>> + !can_mark_large_folio_lazyfree(addr, folio, pte)) >>>>>>> + goto split_large_folio; >>>>>>> + >>>>>>> + /* >>>>>>> + * Avoid unnecessary folio splitting if the large >>>>>>> + * folio is entirely within the given range. >>>>>>> + */ >>>>>>> + folio_clear_dirty(folio); >>>>>>> + folio_unlock(folio); >>>>>>> + for (; addr != next_addr; pte++, addr += PAGE_SIZE) { >>>>>>> + ptent = ptep_get(pte); >>>>>>> + if (pte_young(ptent) || pte_dirty(ptent)) { >>>>>>> + ptent = ptep_get_and_clear_full( >>>>>>> + mm, addr, pte, tlb->fullmm); >>>>>>> + ptent = pte_mkold(ptent); >>>>>>> + ptent = pte_mkclean(ptent); >>>>>>> + set_pte_at(mm, addr, pte, ptent); >>>>>>> + tlb_remove_tlb_entry(tlb, pte, addr); >>>>>>> + } >>>>>> >>>>>> Can we do this in batches? for a CONT-PTE mapped large folio, you are unfolding >>>>>> and folding again. It seems quite expensive. >>> >>> I'm not convinced we should be doing this in batches. We want the initial >>> folio_pte_batch() to be as loose as possible regarding permissions so that we >>> reduce our chances of splitting folios to the min. (e.g. ignore SW bits like >>> soft dirty, etc). I think it might be possible that some PTEs are RO and other >>> RW too (e.g. due to cow - although with the current cow impl, probably not. But >>> its fragile to assume that). Anyway, if we do an initial batch that ignores all >> >> You are correct. I believe this scenario could indeed occur. For instance, >> if process A forks process B and then unmaps itself, leaving B as the >> sole process owning the large folio. The current wp_page_reuse() function >> will reuse PTE one by one while the specific subpage is written. > > Hmm - I thought it would only reuse if the total mapcount for the folio was 1. > And since it is a large folio with each page mapped once in proc B, I thought > every subpage write would cause a copy except the last one? I haven't looked at > the code for a while. But I had it in my head that this is an area we need to > improve for mTHP. wp_page_reuse() will currently reuse a PTE part of a large folio only if a single PTE remains mapped (refcount == 0). -- Cheers, David / dhildenb