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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id u11-20020a056000038b00b0033e456848f4sm9661876wrf.82.2024.03.07.11.48.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 11:48:51 -0800 (PST) Message-ID: <6f0c75cd-bbfd-4b7b-b022-75f7aa577ddd@redhat.com> Date: Thu, 7 Mar 2024 20:48:50 +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: Barry Song <21cnbao@gmail.com>, Ryan Roberts 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> <8f84c7d6-982a-4933-a7a7-3f640df64991@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 5F9DD40019 X-Stat-Signature: w48keuegx8oa3eige7zk5pqnjrwq8xhg X-Rspam-User: X-HE-Tag: 1709840935-162880 X-HE-Meta: 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 dpuhTFSy UPgEKH4bK3WU5SIxNo0JxGPAL5dOhZTKuIhwgQUcd5QVHq5x6c3Uqp4SiIIw7c4opLJEkpYS3xJ4Q5bqhYMVdeP/h8qCAbnvcCem5Wo+sU7bG9LNOFehoXKIxRpxL+uM2hqqLG47rbQG0YN3Ejd0SNQKqnffxQoY+YI2bZZ7S8FoPq+CS0Ui775AdD+olodiCOorWXxaE7iM+4VrGNqW81lkqfVLBKJP8b3HHMbqw00ug3fST+EITGA6Y2svI9rHmCx8TdwEFjp1JMUAMXp7FjLc+FHtn4zecJAJK 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 19:54, Barry Song wrote: > On Fri, Mar 8, 2024 at 12:31 AM Ryan Roberts wrote: >> >> On 07/03/2024 12:01, Barry Song wrote: >>> On Thu, Mar 7, 2024 at 7:45 PM David Hildenbrand wrote: >>>> >>>> On 07.03.24 12:42, Ryan Roberts wrote: >>>>> On 07/03/2024 11:31, David Hildenbrand wrote: >>>>>> On 07.03.24 12:26, Barry Song wrote: >>>>>>> On Thu, Mar 7, 2024 at 7:13 PM Ryan Roberts wrote: >>>>>>>> >>>>>>>> On 07/03/2024 10:54, David Hildenbrand wrote: >>>>>>>>> On 07.03.24 11:54, David Hildenbrand wrote: >>>>>>>>>> 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. >>>>>>> >>>>>>> So sad I am wrong again 😢 >>>>>>> >>>>>>>>>> >>>>>>>>>> wp_page_reuse() will currently reuse a PTE part of a large folio only if >>>>>>>>>> a single PTE remains mapped (refcount == 0). >>>>>>>>> >>>>>>>>> ^ == 1 >>>>>>> >>>>>>> seems this needs improvement. it is a waste the last subpage can >>>>>> >>>>>> My take that is WIP: >>>>>> >>>>>> https://lore.kernel.org/all/20231124132626.235350-1-david@redhat.com/T/#u >>>>>> >>>>>>> reuse the whole large folio. i was doing it in a quite different way, >>>>>>> if the large folio had only one subpage left, i would do copy and >>>>>>> released the large folio[1]. and if i could reuse the whole large folio >>>>>>> with CONT-PTE, i would reuse the whole large folio[2]. in mainline, >>>>>>> we don't have this cont-pte luxury exposed to mm, so i guess we can >>>>>>> not do [2] easily, but [1] seems to be an optimization. >>>>>> >>>>>> Yeah, I had essentially the same idea: just free up the large folio if most of >>>>>> the stuff is unmapped. But that's rather a corner-case optimization, so I did >>>>>> not proceed with that. >>>>>> >>>>> >>>>> I'm not sure it's a corner case, really? - process forks, then both parent and >>>>> child and write to all pages in what was previously a fully & contiguously >>>>> mapped large folio? >>>> >>>> Well, with 2 MiB my assumption was that while it can happen, it's rather >>>> rare. With smaller THP it might get more likely, agreed. >>>> >>>>> >>>>> Reggardless, why is it an optimization to do the copy for the last subpage and >>>>> syncrhonously free the large folio? It's already partially mapped so is on the >>>>> deferred split list and can be split if memory is tight. >>> >>> we don't want reclamation overhead later. and we want memories immediately >>> available to others. >> >> But by that logic, you also don't want to leave the large folio partially mapped >> all the way until the last subpage is CoWed. Surely you would want to reclaim it >> when you reach partial map status? > > To some extent, I agree. But then we will have two many copies. The last > subpage is small, and a safe place to copy instead. Right, it's essentially a simplistic page migration at a point where you know you can safely replace the page (PAE not set, so it cannot be pinned using FOLL_PIN). No rmap walk, no migration entries, no worry about additional page references. -- Cheers, David / dhildenb