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a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1709808642; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=FvC/dt8UyOWUsG/ULL1SlQq4m8R6E00eadXwK5i5a5Y=; b=4o1Kycn3lgYDpIPM3Wf/+54t+JC7uEcHyx11PnEipsznjYn0i5s7SjTqNe6IEfTq1ukR7h oCAsPV06vKpvPQckjo8qx6Fo6CSbYCRgHCq0Jo9JThY4SyrCBSOQmzv87yD9KFFSGJ0PUJ az8czE3zlXWa42gn10AfQgz8fvYeRQw= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6830C1FB; Thu, 7 Mar 2024 02:51:18 -0800 (PST) Received: from [10.1.25.184] (XHFQ2J9959.cambridge.arm.com [10.1.25.184]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 245413F762; Thu, 7 Mar 2024 02:50:39 -0800 (PST) Message-ID: <501c9f77-1459-467a-8619-78e86b46d300@arm.com> Date: Thu, 7 Mar 2024 10:50:37 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free Content-Language: en-GB To: Barry Song <21cnbao@gmail.com> Cc: Lance Yang , david@redhat.com, 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> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 73C5A180006 X-Stat-Signature: ebarir6gtf7s8c6buppobh6qfzbw9ui9 X-HE-Tag: 1709808642-436303 X-HE-Meta: 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 Xoo68ToZ iZ87NaZPgWCXtfpF/SQWRQQbAnDRNWhFz4LRtDx9O20azYqhxEO8xGwJOkQvePz4pHmASoV/XqHROz/zuEBoyC2K8/f4DegwU4ChwJPVTz9rz77Agr6QBkvBNocoErQFEuT/V+a4ynOVEA6xX9ZpXMwIKNXSIk+3PQBzZPfW9QW0rlBjg97vBTNzH2FYgLLD8Tn//NEmNa00IbHeGNF3WRGMa2Wg3BiYxfegED4Tvr3sdZ+jSzNq4Nu2ldU24VrzDVc2xzh/ITy/uLJdk+GXRXAeAcF59kG9nkuB2KVJPW99GfQL5MYBc+YnC/U4E1eLXTKABewqbzyvMKgOM6ydxcwALQfuUx4+SZ37bjF5+93WWRQtehrbKeY7bSkpzKh7CiTAa5NLcgUBKAdnNPkOUS0Pj10aYNnHsWVA6xP6EizU5P65AfcXYIyFtEhD/KxyIIABbWyGoMhNlzW0= 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 07/03/2024 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. > This can > make a part of PTE writable while the others are read-only. > >> that then do this bit as a batch, you will end up smeering all the ptes with >> whatever properties were set on the first pte, which probably isn't right. >> >> I've done a similar conversion for madvise_cold_or_pageout_pte_range() as part >> of my swap-out series v4 (hoping to post imminently, but still working out a >> latent bug that it triggers). I use ptep_test_and_clear_young() in that, which >> arm64 can apply per-pte but avoid doing a contpte unfold/fold. I know you have >> to clear dirty here too, but I think this pattern is preferable. > > nice to know ptep_test_and_clear_young() won't unfold and fold CONT-PTE. > I probably have missed this part of your CONT-PTE series as I was quite busy > with others :-) > >> >> FYI, my swap-out series also halfway-batches madvise_free_pte_range() so that I >> can batch free_swap_and_cache() for the swap entry case. Ideally the work you >> are doing here would be rebased on top of that and plug-in to the approach >> implemented there. (subject to others' views of course). >> >> I'll cc you when I post it. >> >>>> >>>> Thanks for your suggestion. I'll do this in batches in v3. >>>> >>>> Thanks again for your time! >>>> >>>> Best, >>>> Lance >>>> >>>>> >>>>>> + } >>>>>> + folio_mark_lazyfree(folio); >>>>>> + goto next_folio; >>>>>> + >>>>>> +split_large_folio: >>>>>> folio_get(folio); >>>>>> arch_leave_lazy_mmu_mode(); >>>>>> pte_unmap_unlock(start_pte, ptl); >>>>>> @@ -688,13 +736,28 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, >>>>>> err = split_folio(folio); >>>>>> folio_unlock(folio); >>>>>> folio_put(folio); >>>>>> - if (err) >>>>>> - break; >>>>>> - start_pte = pte = >>>>>> - pte_offset_map_lock(mm, pmd, addr, &ptl); >>>>>> - if (!start_pte) >>>>>> - break; >>>>>> - arch_enter_lazy_mmu_mode(); >>>>>> + >>>>>> + /* >>>>>> + * If the large folio is locked or cannot be split, >>>>>> + * we just skip it. >>>>>> + */ >>>>>> + if (err) { >>>>>> +skip_large_folio: >>>>>> + if (next_addr >= end) >>>>>> + break; >>>>>> + pte += (next_addr - addr) / PAGE_SIZE; >>>>>> + addr = next_addr; >>>>>> + } >>>>>> + >>>>>> + if (!start_pte) { >>>>>> + start_pte = pte = pte_offset_map_lock( >>>>>> + mm, pmd, addr, &ptl); >>>>>> + if (!start_pte) >>>>>> + break; >>>>>> + arch_enter_lazy_mmu_mode(); >>>>>> + } >>>>>> + >>>>>> +next_folio: >>>>>> pte--; >>>>>> addr -= PAGE_SIZE; >>>>>> continue; >>>>>> -- >>>>>> 2.33.1 >>>>>> >>> > > Thanks > Barry