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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id i6-20020adfb646000000b0033e033898c5sm20074794wre.20.2024.03.07.03.45.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 03:45:30 -0800 (PST) Message-ID: Date: Thu, 7 Mar 2024 12:45:28 +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-Stat-Signature: w6abrtbh3ixf9mc53qgkxx83oc7iooxd X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: A504BC001D X-Rspam-User: X-HE-Tag: 1709811936-792312 X-HE-Meta: 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 d25iAoq3 cbm/bVy8hY18OjW1mGfe1tTMIxpMVDPgv4FMMm0+G2lNXWNK0jMa0GCvmnIpYWn0mWFovMtT12P4MfBoelVn7INbiiV7wueG47/7kX5cUOABdHdbzmbnNQMI0PsQNnwwD/xpaeZMF+evwNVQg9vWBvdCagrOsSfOuAwDl6hF0nPbJg0pyQ2AkjX9svvY8ExQVMpFIF/q+Ha1gqOcMvQzI50R2K1c45cOp7wSrRJ4i17UN2DEi4/8x6awHJa4Ow8y/g5t/VYN1IMSRbUG4vn9kkK+VijZizvd8YKRQ 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 12:42, 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. At least for 2 MiB THP, it might make sense to make that large folio available immediately again, even without memory pressure. Even compaction would not compact it. -- Cheers, David / dhildenb