From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8675917B401 for ; Sat, 31 Aug 2024 10:29:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725100160; cv=none; b=Jx4sKPa4YHhggEeeKUshfyvLmJ3ip2fmdjo3xP+ycXTtQ/84PVMEzvXib8hD56EJc6nKp2N5tBrWlt1dnSaAjDN1HSrUxl/JHIn5gwJ7GVYAog5B91BsJLPRafoSbMpCvKSy86wHeCp8gFd+g/DmjsaHREx6FgqRHl6eTP++rlA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725100160; c=relaxed/simple; bh=NbHzSpYKnIkTR52Sv91CDXQKCSGg2wPwHy9DXdFvwdk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VfIaEkTwm+yDgu0ea905u2jrcMExbIQwrLqbzAoXHIdEYtfwpzLxctGz7tHPg3Nqw+KkWXdkSNh0dFT7CGkAbCDFaVmRAE/0H0cu6WCbUbxjbhFqeSk8U3kghPN1PZ/NXRjSBeJ0w++EEiv8TTr8665hPELlu5vcsUTxd8QSmTc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=H5YTvYi8; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="H5YTvYi8" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1725100157; h=from:from: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:autocrypt:autocrypt; bh=FrbIITwVcetzf9SQghNcwaEpm4866gXGozxQ3snQfAc=; b=H5YTvYi8NuvC2XoUO3N9UhYFK5pZ6gUtYRHwcUZadOekl3d0GGhsLlDDLgR08xde+RDWf4 skWjRS3WyDSrSnZrht/51Jz+AoDIrCD6959uCthYp+7bPd4/yArsBMUGLBlE8HrZjQUmCc j1VDm4jnqciHXIYN88G+6E7LdnbQI7I= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-161-94Kgu_GpM-q8q_Klc2wCVA-1; Sat, 31 Aug 2024 06:29:15 -0400 X-MC-Unique: 94Kgu_GpM-q8q_Klc2wCVA-1 Received: by mail-wm1-f71.google.com with SMTP id 5b1f17b1804b1-42c7a5563cfso2427495e9.2 for ; Sat, 31 Aug 2024 03:29:15 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1725100154; x=1725704954; h=content-transfer-encoding:in-reply-to:organization:autocrypt :content-language:from:references:cc:to:subject:user-agent :mime-version:date:message-id:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=FrbIITwVcetzf9SQghNcwaEpm4866gXGozxQ3snQfAc=; b=hy/vV4vh1XlroC2x4KKnZ45UH2akGpxTHvKmdzHu+aAoT9UBrwbOuMpg+fk+dlqENn dSvZL8qf8drmjtoP/dBRPRv29sVioImeRsqyK00bqkPyF07U+TngoWQ2RQXHiu4d2pXq Bh0sf0pVIZeGv/kqiMZydOOypJ7cujRc3m0S+RsthuCCYriHmW/YEHf888AxhkGpWIFv W6qok9zFuRDiXzYy8AM6ZnOEGl3hCIdESJR80mumszknUk6pMjjR5FmoPDxCecbLuszC Uyv0yJrSqtf8Od96NQGAKmfwSBW4b3Gjj+1nZSb8p0Vf5oEzEZRzbey6lsqjwHG2PcOx 4DTA== X-Forwarded-Encrypted: i=1; AJvYcCVVKj23sOZA2HgK0aRDkCPDI/s7VoB+SxJSp0Z+F3NvIj5Cfiga2s6KUHp/SjhhYEKyybPj+LuLYoLtCnY=@vger.kernel.org X-Gm-Message-State: AOJu0YxYsDek9TmyXupm7iOiBxyC3OW0V0HVke1k7/oOeCiQ05KH8N7z qB9tvFoN50WoyN2rb9hDjCB17C88TrS4uSBkn8s8qAtAK6KuXbUTDAW/JP8+2gDV/tUAKesuHQa FntqX2//u53dPKWQdKJ8K5xvf+Zr344AjpxsnoqVYXtG+ry7hbiWe+sS72bLcuQ== X-Received: by 2002:a05:600c:4585:b0:426:51dc:f6cd with SMTP id 5b1f17b1804b1-42bb01bfbdemr74410305e9.18.1725100154282; Sat, 31 Aug 2024 03:29:14 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFAM5aUb/RrIahwDOIk1OJwl1CK2O+IP3X2EUvQEn/8oV4OgVRuvCJ6WkztHU1d37U5gf49Sg== X-Received: by 2002:a05:600c:4585:b0:426:51dc:f6cd with SMTP id 5b1f17b1804b1-42bb01bfbdemr74409925e9.18.1725100153379; Sat, 31 Aug 2024 03:29:13 -0700 (PDT) Received: from [192.168.3.141] (p5b0c6ee6.dip0.t-ipconnect.de. [91.12.110.230]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-374b9d54f98sm2702991f8f.69.2024.08.31.03.29.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 31 Aug 2024 03:29:12 -0700 (PDT) Message-ID: <36933711-ae0f-468c-93bd-d6a67d974c9d@redhat.com> Date: Sat, 31 Aug 2024 12:29:11 +0200 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 RFC] mm: entirely reuse the whole anon mTHP in do_wp_page To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Barry Song , Chuanhua Han , Baolin Wang , Ryan Roberts , Zi Yan , Chris Li , Kairui Song , Kalesh Singh , Suren Baghdasaryan References: <20240831092339.66085-1-21cnbao@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 31.08.24 12:21, Barry Song wrote: > On Sat, Aug 31, 2024 at 10:07 PM David Hildenbrand wrote: >> >> On 31.08.24 11:55, Barry Song wrote: >>> On Sat, Aug 31, 2024 at 9:44 PM David Hildenbrand wrote: >>>> >>>> On 31.08.24 11:23, Barry Song wrote: >>>>> From: Barry Song >>>>> >>>>> On a physical phone, it's sometimes observed that deferred_split >>>>> mTHPs account for over 15% of the total mTHPs. Profiling by Chuanhua >>>>> indicates that the majority of these originate from the typical fork >>>>> scenario. >>>>> When the child process either execs or exits, the parent process should >>>>> ideally be able to reuse the entire mTHP. However, the current kernel >>>>> lacks this capability and instead places the mTHP into split_deferred, >>>>> performing a CoW (Copy-on-Write) on just a single subpage of the mTHP. >>>>> >>>>> main() >>>>> { >>>>> #define SIZE 1024 * 1024UL >>>>> void *p = malloc(SIZE); >>>>> memset(p, 0x11, SIZE); >>>>> if (fork() == 0) >>>>> exec(....); >>>>> /* >>>>> * this will trigger cow one subpage from >>>>> * mTHP and put mTHP into split_deferred >>>>> * list >>>>> */ >>>>> *(int *)(p + 10) = 10; >>>>> printf("done\n"); >>>>> while(1); >>>>> } >>>>> >>>>> This leads to two significant issues: >>>>> >>>>> * Memory Waste: Before the mTHP is fully split by the shrinker, >>>>> it wastes memory. In extreme cases, such as with a 64KB mTHP, >>>>> the memory usage could be 64KB + 60KB until the last subpage >>>>> is written, at which point the mTHP is freed. >>>>> >>>>> * Fragmentation and Performance Loss: It destroys large folios >>>>> (negating the performance benefits of CONT-PTE) and fragments memory. >>>>> >>>>> To address this, we should aim to reuse the entire mTHP in such cases. >>>>> >>>>> Hi David, >>>>> >>>>> I’ve renamed wp_page_reuse() to wp_folio_reuse() and added an >>>>> entirely_reuse argument because I’m not sure if there are still cases >>>>> where we reuse a subpage within an mTHP. For now, I’m setting >>>>> entirely_reuse to true only for the newly supported case, while all >>>>> other cases still get false. Please let me know if this is incorrect—if >>>>> we don’t reuse subpages at all, we could remove the argument. >>>> >>>> See [1] I sent out this week, that is able to reuse even without >>>> scanning page tables. If we find the the folio is exclusive we could try >>>> processing surrounding PTEs that map the same folio. >>>> >>>> [1] https://lkml.kernel.org/r/20240829165627.2256514-1-david@redhat.com >>> >>> Great! It looks like I missed your patch again. Since you've implemented this >>> in a better way, I’d prefer to use your patchset. >> >> I wouldn't say better, just more universally. And while taking care of >> properly sync'ing the mapcount vs. refcount :P >> >>> >>> I’m curious about how you're handling ptep_set_access_flags_nr() or similar >>> things because I couldn’t find the related code in your patch 10/17: >>> >>> [PATCH v1 10/17] mm: COW reuse support for PTE-mapped THP with CONFIG_MM_ID >>> >>> Am I missing something? >> >> The idea is to keep individual write faults as fast as possible. So the >> patch set keeps it simple and only reuses a single PTE at a time, >> setting that one PAE and mapping it writable. > > I got your point, thanks! as anyway the mTHP has been exclusive, > so the following nr-1 minor page faults will set their particular PTE > to writable one by one. Yes, assuming you would get these page faults, and assuming you would get them in the near future. > >> >> As the patch states, it might be reasonable to optimize some cases, >> maybe also only on some architectures. For example to fault-around and >> map the other ones writable as well. It might not always be desirable >> though, especially not for larger folios. > > as anyway, the mTHP has been entirely exclusive, setting all PTEs > directly to writable should help reduce nr - 1 minor page faults and > ideally help reduce CONTPTE unfold and fold? Yes, doing that on CONTPTE granularity would very likely make sense. For anything bigger than that, I am not sure. Assuming we have a 1M folio mapped by PTEs. Trying to fault-around in aligned CONTPTE granularity likely makes sense. Bigger than that, I am not convinced. > > What is the downside to doing that? I also don't think mapping them > all together will waste memory? No, it's all about increasing the latency of individual write faults. -- Cheers, David / dhildenb