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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id n9-20020a05600c3b8900b0041310142e37sm1894088wms.47.2024.03.07.03.31.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 03:31:58 -0800 (PST) Message-ID: Date: Thu, 7 Mar 2024 12:31:57 +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-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 3144F120008 X-Stat-Signature: tpcz4n7apq6ii5aeddps3ffqxoc3qgrh X-HE-Tag: 1709811122-639643 X-HE-Meta: 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 ms6YiF8o Rw8XheiRFiljec2ykY0gZr3dLprEibZSmgaH0n1T76DR9/geAYnFWtkqWGwg9d3x46fuWibo1vcSCBvxzY1SK7o3LoCSXuQlflDj/7tPgDLtiiThqBcn86qUCPesAW30Ye6tal/fD0aijCjFI2IOBFs2K5vRCmS0LGpjV3fZzP+wke3pj+ebHytLZHT5Dhh6eAbTGYCPPYSeKK1HpHLSHaaNWVwiJKzKhIh14Uj7Wis7abpX0/MwFOtM4nYNvR5+VJiMbdg3seW0N8r2401serfMI6WVUKrHtF9dN 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 12:26, 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. -- Cheers, David / dhildenb