From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 613FA85641 for ; Thu, 7 Mar 2024 10:50:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709808645; cv=none; b=IYV5URDTJbYxxZliMWxCuhX+zNjC1e5vOtQe6nkfvDIeYYQG3Ce8DJ3L8KzjVw8hswIKBYxhv/q8693mo2JCGLs/H8SdKOX+ZENiuJeNhWsrjfCCnTJsP3u7In3h3PHumpNhXcRTgKXfUL7gka8ySoJFC0W/ccgDdko+bZ8+baI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709808645; c=relaxed/simple; bh=dLBWtzXmuxNb9Ha8327cbmKbIwdaV8Yc+gOYw2Ai4/0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gB/OgRJLG26bAT83ZY2K8L/tVIq4BLlkid0j75tqhcQ0pTq7fYun7t9A6zYQfunuUcmGL6t5aCyUxTbsRs9BaIMN09vSbfwnWhaI452L/FTBkaDgZ+4TPnLqb1Gp3uckIsOOidpPsuUHx4n1pSXq+sDFU40uVikfh7CZKZsV76k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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