From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id E234DC3ABAA for ; Fri, 2 May 2025 15:39:19 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 16FB66B000A; Fri, 2 May 2025 11:39:18 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0F9DA6B008A; Fri, 2 May 2025 11:39:18 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E8E746B008C; Fri, 2 May 2025 11:39:17 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id C26F46B000A for ; Fri, 2 May 2025 11:39:17 -0400 (EDT) Received: from smtpin19.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay01.hostedemail.com (Postfix) with ESMTP id 860FF1D10F6 for ; Fri, 2 May 2025 15:39:18 +0000 (UTC) X-FDA: 83398376796.19.3C0C0E9 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf24.hostedemail.com (Postfix) with ESMTP id E8E1D180008 for ; Fri, 2 May 2025 15:39:15 +0000 (UTC) Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Ca6fWGC9; dmarc=pass (policy=quarantine) header.from=redhat.com; spf=pass (imf24.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1746200356; a=rsa-sha256; cv=none; b=xetTZcfkU8TCAfEh9br2baYuK7jMjkBBzpbq+r14FJvUlfM/+jRBxSino6nEnhegtbOBZN L4PLXYsILeHbs/V0rbWUPHnae/4a4ZCVknlrJ6ckSY+JWLf6xXxKG7A3o+1hgYeju+/cHF 5Fc22ijm8W+Y2rrbstyGHXoV9LE6Y9A= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Ca6fWGC9; dmarc=pass (policy=quarantine) header.from=redhat.com; spf=pass (imf24.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1746200356; 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:dkim-signature; bh=RSbcHiqENjgdwQ8aS6PFFw/+bfZ9pm2Lqqqi4VVjILQ=; b=R37pWDFBfDNFbcDmXBhwujw4olX/M7u6y4y7YzM89Pe8gleUatbwMvxOJYSOebz5aJbjad pV0FSfJ25i1Se7jc6ouzAnwPbOZb1JgcKl4oAhPyZQyav4FGOurn89YBQkyrlIHSe/OPFk wjKU4iuce2tKBfBIqqzwNxyBzm0SNTA= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1746200355; 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=RSbcHiqENjgdwQ8aS6PFFw/+bfZ9pm2Lqqqi4VVjILQ=; b=Ca6fWGC9ieT/CpbLiuzhiqhwtcPQ7B9jkwFX/kBaqB+2hcerl0X93G1EeErbBnFpJEqzi8 WY3pvw1XJt4X2z4bC2WtDRGEiSGQtudfvSnuwjzOPzB2NiyOfCYwICIKec8TQmrGyxkuQ5 i5KPGE5yMJVtym1HH0xPpKyZgmbEPBQ= Received: from mail-wm1-f72.google.com (mail-wm1-f72.google.com [209.85.128.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-155-EcU7PvXUPgi1eAGioRausA-1; Fri, 02 May 2025 11:39:14 -0400 X-MC-Unique: EcU7PvXUPgi1eAGioRausA-1 X-Mimecast-MFC-AGG-ID: EcU7PvXUPgi1eAGioRausA_1746200353 Received: by mail-wm1-f72.google.com with SMTP id 5b1f17b1804b1-43d01024089so13587105e9.1 for ; Fri, 02 May 2025 08:39:13 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1746200353; x=1746805153; 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=RSbcHiqENjgdwQ8aS6PFFw/+bfZ9pm2Lqqqi4VVjILQ=; b=cq5BNISvnpl+T2AxPtoD66WXNuY8DiTBSuFhNpLlKKJQd7rHXY8JoMj8MxKY5H/ZvR qzNiku0fpo/wL0mtSFwgN2uPdq3pHwvpksu8AS/O33Sbo6QF0cJQHCd7N6ozrMDqZrKe ZGVsLKVy4UT/Gcz0ydwJ0WycmLfT0Or/On10/AUlMBZ4uCd4pd6o6mzSjTG+KVUiGsgx t/rS+XjtUKRZQJcrAeEbk8Yvtd2TW11gYKiR89IYLzzkiB9/CPQDlkt9/mq2AB/gVwel H7mHw+8iqD44Id02byJL3jxUHeNzwX94gx7rqjbqAUBihE8/Lemtsnpu8e2pIDmBC4bG 3VqQ== X-Forwarded-Encrypted: i=1; AJvYcCVkqEhQfUFYinqWbVmg6JvGbwOoGGOeBlXGGRhWD9N9ziH737yNfWChfqBQUIL1kntzbfOxOjh69Q==@kvack.org X-Gm-Message-State: AOJu0YzSaJGenkKLODQe78nWnoVtu4HIsnFz9OWZQx8L6/Q7PyR/9ake vGQmUcnwCGbnBsiO4EaIPygDKF3BAJen9dEBMNAU/DZ7NeWXRiW9MgmbdjSDWY6unEVACLejppE 52JJNj2iFrhB6+SvTkqNaexq3JHTfzbkm3hqenrhdki7fau6y X-Gm-Gg: ASbGncvgtXTziLJBDuyIZ3fyYznm09J3qdO83cLvFH9mb0sLuB6fSp5xd7Ye0UhRI/P M4xawbYH2ryrcJkNZoJzZZnf0E0Xshj5r22UynXQrr6AnFO/qhg213qcA7OQAaxee1ipeQlBHce hqQ7gBZ+noBP9G9WPij/naOxFBd5khFpu3XffEo27PEcEgavspCcxMHS/8t6hBJmUjJBGw73U9q /oz+kc8zdvUKJYb2onx5ZG3zQskWCR/hL89O68xmmdrT5tHVrhicAQx54TxnUqkFKqVPgHIDGyO 9FYvKnDPCD72oSLQKCgpAwKtWZro0hgR+ijhUL96H+G31s9QI8Bi5GtAvg30A3FieNzY3JHINln iVWZu/Xp5FnjyJLmjVRx2Pvmuyyv3FKJaFWIhrx8= X-Received: by 2002:a05:6000:2507:b0:39a:cb5b:78fe with SMTP id ffacd0b85a97d-3a099addcfemr2359112f8f.34.1746200352697; Fri, 02 May 2025 08:39:12 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHnhW0Pnrk5h6Vk74Y3f0qi3YSQtYQ/2x2XGBwUbyQAt5gxgGilCqo0Szci655DbDOYPQhlYA== X-Received: by 2002:a05:6000:2507:b0:39a:cb5b:78fe with SMTP id ffacd0b85a97d-3a099addcfemr2359092f8f.34.1746200352184; Fri, 02 May 2025 08:39:12 -0700 (PDT) Received: from ?IPV6:2003:cb:c713:d600:afc5:4312:176f:3fb0? (p200300cbc713d600afc54312176f3fb0.dip0.t-ipconnect.de. [2003:cb:c713:d600:afc5:4312:176f:3fb0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441b2af2a5fsm96540615e9.20.2025.05.02.08.39.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 02 May 2025 08:39:11 -0700 (PDT) Message-ID: Date: Fri, 2 May 2025 17:39:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 07/12] khugepaged: add mTHP support To: Lorenzo Stoakes Cc: Jann Horn , Nico Pache , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, akpm@linux-foundation.org, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, ryan.roberts@arm.com, willy@infradead.org, peterx@redhat.com, ziy@nvidia.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kirill.shutemov@linux.intel.com, aarcange@redhat.com, raquini@redhat.com, dev.jain@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, Liam.Howlett@oracle.com References: <20250428181218.85925-1-npache@redhat.com> <20250428181218.85925-8-npache@redhat.com> <35fc73ea-39f7-4d60-9d78-d700b8ef6ff6@lucifer.local> From: David Hildenbrand 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: <35fc73ea-39f7-4d60-9d78-d700b8ef6ff6@lucifer.local> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Lc0zlfhZdZzYMNmGoN-jQfDfFHsMvMO025J0j33GYG8_1746200353 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: E8E1D180008 X-Rspamd-Server: rspam04 X-Stat-Signature: pi5ci44mqwzuarqo8yudh9a5kkdyhuea X-HE-Tag: 1746200355-588626 X-HE-Meta: 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 ruJXggA4 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 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 02.05.25 17:24, Lorenzo Stoakes wrote: > On Fri, May 02, 2025 at 05:18:54PM +0200, David Hildenbrand wrote: >> On 02.05.25 14:50, Jann Horn wrote: >>> On Fri, May 2, 2025 at 8:29 AM David Hildenbrand wrote: >>>> On 02.05.25 00:29, Nico Pache wrote: >>>>> On Wed, Apr 30, 2025 at 2:53 PM Jann Horn wrote: >>>>>> >>>>>> On Mon, Apr 28, 2025 at 8:12 PM Nico Pache wrote: >>>>>>> Introduce the ability for khugepaged to collapse to different mTHP sizes. >>>>>>> While scanning PMD ranges for potential collapse candidates, keep track >>>>>>> of pages in KHUGEPAGED_MIN_MTHP_ORDER chunks via a bitmap. Each bit >>>>>>> represents a utilized region of order KHUGEPAGED_MIN_MTHP_ORDER ptes. If >>>>>>> mTHPs are enabled we remove the restriction of max_ptes_none during the >>>>>>> scan phase so we dont bailout early and miss potential mTHP candidates. >>>>>>> >>>>>>> After the scan is complete we will perform binary recursion on the >>>>>>> bitmap to determine which mTHP size would be most efficient to collapse >>>>>>> to. max_ptes_none will be scaled by the attempted collapse order to >>>>>>> determine how full a THP must be to be eligible. >>>>>>> >>>>>>> If a mTHP collapse is attempted, but contains swapped out, or shared >>>>>>> pages, we dont perform the collapse. >>>>>> [...] >>>>>>> @@ -1208,11 +1211,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >>>>>>> vma_start_write(vma); >>>>>>> anon_vma_lock_write(vma->anon_vma); >>>>>>> >>>>>>> - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address, >>>>>>> - address + HPAGE_PMD_SIZE); >>>>>>> + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, _address, >>>>>>> + _address + (PAGE_SIZE << order)); >>>>>>> mmu_notifier_invalidate_range_start(&range); >>>>>>> >>>>>>> pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */ >>>>>>> + >>>>>>> /* >>>>>>> * This removes any huge TLB entry from the CPU so we won't allow >>>>>>> * huge and small TLB entries for the same virtual address to >>>>>> >>>>>> It's not visible in this diff, but we're about to do a >>>>>> pmdp_collapse_flush() here. pmdp_collapse_flush() tears down the >>>>>> entire page table, meaning it tears down 2MiB of address space; and it >>>>>> assumes that the entire page table exclusively corresponds to the >>>>>> current VMA. >>>>>> >>>>>> I think you'll need to ensure that the pmdp_collapse_flush() only >>>>>> happens for full-size THP, and that mTHP only tears down individual >>>>>> PTEs in the relevant range. (That code might get a bit messy, since >>>>>> the existing THP code tears down PTEs in a detached page table, while >>>>>> mTHP would have to do it in a still-attached page table.) >>>>> Hi Jann! >>>>> >>>>> I was under the impression that this is needed to prevent GUP-fast >>>>> races (and potentially others). >>> >>> Why would you need to touch the PMD entry to prevent GUP-fast races for mTHP? >>> >>>>> As you state here, conceptually the PMD case is, detach the PMD, do >>>>> the collapse, then reinstall the PMD (similarly to how the system >>>>> recovers from a failed PMD collapse). I tried to keep the current >>>>> locking behavior as it seemed the easiest way to get it right (and not >>>>> break anything). So I keep the PMD detaching and reinstalling for the >>>>> mTHP case too. As Hugh points out I am releasing the anon lock too >>>>> early. I will comment further on his response. >>> >>> As I see it, you're not "keeping" the current locking behavior; you're >>> making a big implicit locking change by reusing a codepath designed >>> for PMD THP for mTHP, where the page table may not be exclusively >>> owned by one VMA. >> >> That is not the intention. The intention in this series (at least as we >> discussed) was to not do it across VMAs; that is considered the next logical >> step (which will be especially relevant on arm64 IMHO). >> >>> >>>>> As I familiarize myself with the code more, I do see potential code >>>>> improvements/cleanups and locking improvements, but I was going to >>>>> leave those to a later series. >>>> >>>> Right, the simplest approach on top of the current PMD collapse is to do >>>> exactly what we do in the PMD case, including the locking: which >>>> apparently is no completely the same yet :). >>>> >>>> Instead of installing a PMD THP, we modify the page table and remap that. >>>> >>>> Moving from the PMD lock to the PTE lock will not make a big change in >>>> practice for most cases: we already must disable essentially all page >>>> table walkers (vma lock, mmap lock in write, rmap lock in write). >>>> >>>> The PMDP clear+flush is primarily to disable the last possible set of >>>> page table walkers: (1) HW modifications and (2) GUP-fast. >>>> >>>> So after the PMDP clear+flush we know that (A) HW can not modify the >>>> pages concurrently and (B) GUP-fast cannot succeed anymore. >>>> >>>> The issue with PTEP clear+flush is that we will have to remember all PTE >>>> values, to reset them if anything goes wrong. Using a single PMD value >>>> is arguably simpler. And then, the benefit vs. complexity is unclear. >>>> >>>> Certainly something to look into later, but not a requirement for the >>>> first support, >>> >>> As I understand, one rule we currently have in MM is that an operation >>> that logically operates on one VMA (VMA 1) does not touch the page >>> tables of other VMAs (VMA 2) in any way, except that it may walk page >>> tables that cover address space that intersects with both VMA 1 and >>> VMA 2, and create such page tables if they are missing. >> >> Yes, absolutely. That must not happen. And I think I raised it as a problem >> in reply to one of Dev's series. >> >> If this series does not rely on that it must be fixed. >> >>> >>> This proposed patch changes that, without explicitly discussing this >>> locking change. >> >> Yes, that must not happen. We must not zap a PMD to temporarily replace it >> with a pmd_none() entry if any other sane page table walker could stumble >> over it. >> >> This includes another VMA that is not write-locked that could span the PMD. > > I feel like we should document these restrictions somewhere :) > > Perhaps in a new page table walker doc, or on the > https://origin.kernel.org/doc/html/latest/mm/process_addrs.html page. > > Which sounds like I'm volunteering myself to do so doesn't it... > > [adds to todo...] :) that would be nice. Yeah, I mean this is very subtle, but essentially: unless you exclude all page table walkers (well, okay, HW and GUP-fast are a bit special), temporarily replacing a present PTE by pte_none() will cause trouble. Same for PMDs. That's also one of the problems of looking into only using the PTE table lock and not any other heavy-weight locking in khugepaged. As soon as we temporarily unlock the PTE table, but have to temporarily unmap the PTEs (->pte_none()) as well, we need something else (e.g., migration entry) to tell other page table walkers to back off and wait for us to re-take the PTL lock and finish. Zapping must still be able to continue, at which point it gets hairy ... ... which is why I'm hoping that we can play with that once we have the basics running. With the intend of reusing the existing locking scheme -> single VMA spans the PMD. -- Cheers, David / dhildenb