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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id bx5-20020a5d5b05000000b0033e103eaf5bsm20165497wrb.115.2024.03.07.03.17.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 03:17:26 -0800 (PST) Message-ID: Date: Thu, 7 Mar 2024 12:17:25 +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> <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: rspam09 X-Rspamd-Queue-Id: D9D5620011 X-Stat-Signature: rbj83q4fu9jbc8wuxrs3db9t64sm8sok X-Rspam-User: X-HE-Tag: 1709810250-807466 X-HE-Meta: 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 DzAAXKQo SuAlZmtp5bAcKEwCDWXXHxnCxiaxhEMa/sckMAyAnJKCvUkyJYeEXz+3F7xcOPxQPwd6DmocG1F2YEle/01PCNfh1M1PAFyLBqvHvGpWzM0sHX/hHSkHxduz6s8Oku+uexCVX+Ig/3CYdtIr8ypEczwfw0ilfX8IuIule5t0/bHkbdx0o6FKt35NHrq3TENiHd5O3DGl1eionWPzzowQhKjv1EpzLkcYFmBkiAVqpYGZf1k4C1FkryXYKdfi3V93zFw/gQ317yL00CDLTT135QkqIl597Ba3XsRsUubEqH2yi0M6M0LWWk/cEEYiJiiihVWsOH11bVF3mE860tu8Sdy08NgtoVCUGfy6w3rrxanHZ9jqDNF85BlvOBVTbW/yJeLXle0t6yrXx49GoV3lf3oVzQHeRECnbJPA7fVuM0epMzMK1KAiizG6PEXwXGEZweJPokQGWjE9dwbNfQN1g/YX1UAAMBzIL4jTlUM0MdqRQJ5NMwLpTfIQ7YdYntYiKlP4OsRHmx7PLZ/pGbF8FpPKCfn8WLlzZDZ/u14sGjrnqbpVAeLJj1oJJ1MuM7nBL9HU6wZh5dP/aeAl528LDqqjqZmo+/Ja7jyEq8uRITD3FfvTOTxekKb6bO/F75ndjDcVEPeP/eoShPBGN3+O58thAYUig0rrcDSa7SST4JoZB3D4Ol8TkUtt2HOm3n9rzFEVuI9eHPjorh/ZmlT/XtH+a+0MEscjSDF4P 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 12:13, 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. >>> >>> wp_page_reuse() will currently reuse a PTE part of a large folio only if >>> a single PTE remains mapped (refcount == 0). >> >> ^ == 1 > > Ahh yes. That's what I meant. I got the behacviour vagulely right though. > > Anyway, regardless, I'm not sure we want to batch here. Or if we do, we want to > batch function that will only clear access and dirty. We likely want to detect a folio batch the "usual" way (as relaxed as possible), then do all the checks (#pte == folio_mapcount() under folio lock), and finally batch-update the access and dirty only. -- Cheers, David / dhildenb