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 88D14C369D5 for ; Wed, 23 Apr 2025 19:30:18 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 861A66B0005; Wed, 23 Apr 2025 15:30:16 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7E8386B0007; Wed, 23 Apr 2025 15:30:16 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 611846B0008; Wed, 23 Apr 2025 15:30:16 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 402C46B0005 for ; Wed, 23 Apr 2025 15:30:16 -0400 (EDT) Received: from smtpin10.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 5976CBAFAC for ; Wed, 23 Apr 2025 19:30:17 +0000 (UTC) X-FDA: 83366299674.10.F182C00 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf29.hostedemail.com (Postfix) with ESMTP id C7A86120018 for ; Wed, 23 Apr 2025 19:30:14 +0000 (UTC) Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=fkfC7fxW; dmarc=pass (policy=quarantine) header.from=redhat.com; spf=pass (imf29.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=1745436615; a=rsa-sha256; cv=none; b=7Y/5j7z3IW5E2RLQYx84n2QS5BaORM70eNGGHEODs2SzKzT/xpdUpRTl7bh+JM04kG+NNV 7MtnA8L9Zjz/HxceDUpD+KDv6wjq7+9Kf3s7mFXFwbQumnxfyGvx3e7KuaDIdb4Ws4fwUD lISMM/hZpqaAQVlJR0kUFW8qFtQIK8M= ARC-Authentication-Results: i=1; imf29.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=fkfC7fxW; dmarc=pass (policy=quarantine) header.from=redhat.com; spf=pass (imf29.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=1745436615; 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=PItQ7KszmU2zVpKOnuHtjt42DwYSHkk8jr7NtA39BJo=; b=LKZi/4QGJ819g89cLBPpXz64EQO5K0xRtVftc97lJkh83He1bk3D5B+vvZmXBFiuA1mowg k2uNYFutLv+0eakp8kl/w8GG6vzGmF3AWNiXCOor57C15F2WJcdzFbP4rmJGuefTxDclt4 mYD1sIImWFAgfCfQVF0LY4jWqpHPWAk= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1745436614; 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=PItQ7KszmU2zVpKOnuHtjt42DwYSHkk8jr7NtA39BJo=; b=fkfC7fxW5YTd00Ob0AWNa6zFHW3661ofhhFKdbxYpRkS6VSZ9fWYgFIlJsUXarfY6RFJUc YZN4LVhayV0wwXthUgpxLBwf4hF/P4WCEYg83rg1YP94tNZbnJr0mWf7mY7YD7n5hAVHeM 03S95MzHOK12fa5ylr07t5+4SFhmKj0= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-563-Wsg45g-ENPa7fso9zkr8Lg-1; Wed, 23 Apr 2025 15:30:12 -0400 X-MC-Unique: Wsg45g-ENPa7fso9zkr8Lg-1 X-Mimecast-MFC-AGG-ID: Wsg45g-ENPa7fso9zkr8Lg_1745436611 Received: by mail-wm1-f70.google.com with SMTP id 5b1f17b1804b1-43cfda30a3cso856615e9.3 for ; Wed, 23 Apr 2025 12:30:12 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1745436611; x=1746041411; 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=PItQ7KszmU2zVpKOnuHtjt42DwYSHkk8jr7NtA39BJo=; b=dasUdzbSI35dc1imSeKoMKP6n2kAJw8uX1BG6FApRCqLNWY3LGEO24zxO3NXHdHvTl YqYqoMpoWO9iJfsHIjXGL8T5ugHQBwvRJqaSFjBoxXfLk43nusKFOjlZL08TLvFHERqp HCJHVmL7XUJkt2bXFIbT/QWX9Vvy/rB97nrmo0+NiekjdCDmgstjGcgwoGd75LH3/EXC tWy5UxxyXAfM8tUD7chedH8jKg1tiG5ysK2fNcViqMIkTMLe8XzgweBAcefAcjebj4oX JC8II0jqm6FEJ++cTx/jrFCuTXAMGZ3b5AN+Mszmeu2dWNNjXZZ6b1av1DKG0NiJjrJz 5wEQ== X-Forwarded-Encrypted: i=1; AJvYcCXAcffAQMn83T3pIWL/L1EZFA6uJvJ2OyD8vjmETeQhWYIKUcWMQ2KMV6haLeFOBKAADYd5R9Bmeg==@kvack.org X-Gm-Message-State: AOJu0YwEcfvGAWruQWosjmFS0CdxHAsQsqSO2mNgLGRC7Y4Q4VDfVuFW jpRGCbPsjSOPbMdmLr7SLfMOrwn0yTPtypAshfhF806ABxhxSMFD2tQmqoce4VZIANOb9I05DNo ziAF99mQE4PL8Ufl8x7fo8DXYkJkna0jUY11GUCBy67TZEcHv X-Gm-Gg: ASbGncubM9MmLUBPdLjVhIdNG578Woeg6K5ZZ70q1nA8oTV/jfY5RP1EqxSEF6PSIA6 kkthlB/ibme9YZFzLTQHgRDbjWd19p0KGui+Mk9+ZXM8omPWHESQiqZXmDEY0g0IIQ0/Zt09Pyn tOu7ZcHFZUD4LGXGckhMtg+nWNiIahIsyr8BwZdcLWXlJGhMpkNDMJtTBfPmKtQfw68LE8c8kCe 6i2Vz6i8uae40DaBN57BE5Hp4sWEbYEKDqvbn2TedOfCAVp1hZe8AVB7c1F1X4PeV1Hyr7vHaEb 7hQ1xLjwSYyH3FZa577gGXq+/0q5wtcSwbZV15oWby6c/Ek+MpotG+PKv5ZcfbkyUJWAX27ISqF WPHMFgA/gM7XSq75ATPDkcIHRA1r+IY1Lnv0X2rA= X-Received: by 2002:a05:600c:4ecc:b0:43c:eeee:b70a with SMTP id 5b1f17b1804b1-4406ac0aec8mr152970775e9.22.1745436611259; Wed, 23 Apr 2025 12:30:11 -0700 (PDT) X-Google-Smtp-Source: AGHT+IF1mjW2K2VIHahK7ukBTM9rHWQgXcITpHuf/BlpRGgBPX3GHnGPhT1uWjmZk3Og2lSYCJjoPg== X-Received: by 2002:a05:600c:4ecc:b0:43c:eeee:b70a with SMTP id 5b1f17b1804b1-4406ac0aec8mr152970485e9.22.1745436610824; Wed, 23 Apr 2025 12:30:10 -0700 (PDT) Received: from ?IPV6:2003:cb:c740:2c00:d977:12ba:dad2:a87f? (p200300cbc7402c00d97712badad2a87f.dip0.t-ipconnect.de. [2003:cb:c740:2c00:d977:12ba:dad2:a87f]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39efa4931acsm19941044f8f.72.2025.04.23.12.30.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Apr 2025 12:30:10 -0700 (PDT) Message-ID: Date: Wed, 23 Apr 2025 21:30:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] kernel/fork: only call untrack_pfn_clear() on VMAs duplicated for fork() To: Lorenzo Stoakes Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, Andrew Morton , Ingo Molnar , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Borislav Petkov , Rik van Riel , "H. Peter Anvin" , Linus Torvalds References: <20250422144942.2871395-1-david@redhat.com> <45b26168-93c9-4170-b7e1-4b51078a04f6@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: <45b26168-93c9-4170-b7e1-4b51078a04f6@lucifer.local> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 8q8end-cGN-S-p1TPsecu3W_EnVO0pSE8yyHhLAfJAY_1745436611 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: C7A86120018 X-Stat-Signature: 8gga95ijbcf36nb6ix7htq8nixtzz1tn X-Rspam-User: X-HE-Tag: 1745436614-229803 X-HE-Meta: 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 P6Wljxw7 ssennmRIlSxpaGEU3Fdu9J9AOlLlubOXgGBH3LEiwah6cmXHbwAD2aylckW+tGRpJ4gltR59EFdyHfEgajmASD0myPCASSbc/CxU5OQnCGbRwxrWcTc1AEBjmDp1nrC1WkWDwWZ2ek+aA1ZffX2o6HRg0ykdBcOMQGIlKNW9mdZd940NaK20dTR6iIOZ9cyIW2RlXX3fqS50f2ZUEMJJ1eDHcluhZxuz9Zn3/KPc9NuziBb0us9RQIClPTZXp6iGeNNnlzsqWxoL3TeNN0EPeeXfR02/k+OHVAIBPraVZm9ts9yEhdN52AOJTJYvMOuwrkP5oGJYvonpp+3eDYe+NJqrb4H1L5ktw0gSiL/85C8xndpQy1qpE9vvSmsEn4UtoVbqSbswXFlW2J509zZlKoa3/yswfY2gcx7KVQ+VJ4jkwF5Q6/u1XEKF/Ob9j/MyQd5oUbHHuH2dyd75SJxKlE++bZCsrQBNfhxZe 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 23.04.25 16:41, Lorenzo Stoakes wrote: > +cc Liam for the vma things, and because he adores PAT stuff ;) > > On Tue, Apr 22, 2025 at 04:49:42PM +0200, David Hildenbrand wrote: >> Not intuitive, but vm_area_dup() located in kernel/fork.c is not only >> used for duplicating VMAs during fork(), but also for duplicating VMAs >> when splitting VMAs or when mremap()'ing them. > > Ugh this sucks, I really want to move a bunch of this stuff out of the fork > code. we have some really nasty overlap there. > > This definitely needs to be separate out. Perhaps I can take a look at > that... Yes, it should likely live in ... vma.c ? :) > >> >> VM_PFNMAP mappings can at least get ordinarily mremap()'ed (no change in >> size) and apparently also shrunk during mremap(), which implies >> duplicating the VMA in __split_vma() first. > > Yes, it appears we only disallow VM_PFNMAP on a remap if it is MREMAP_FIXED > (implies MREMAP_MAYMOVE) to a new specific address _and_ we _increase_ the > size of the VMA. > > (as determined by vrm_implies_new_addr(), with resize_is_valid() explicitly > disallowing MREMAP_DONTUNMAP). > > Makes sense as VM_PFNMAP implies we map non-vm_normal_folio() stuff, which > can't be faulted in, and thus we can't have unfaulted backing for it, but > we can shrink safely. > >> >> In case of ordinary mremap() (no change in size), we first duplicate the >> VMA in copy_vma_and_data()->copy_vma() to then call untrack_pfn_clear() on >> the old VMA: we effectively move the VM_PAT reservation. So the >> untrack_pfn_clear() call on the new VMA duplicating is wrong in that >> context. >> > > OK so we do: > > copy_vma_and_data() > -> copy_vma() > -> vm_area_dup() > -> untrack_pfn_clear(new vma) > > And: > > copy_vma_and_data() > -> untrack_pfn_clear(old vma) > > So we end up with... neither tracked. Fun. > > Agreed this is incorrect. > >> Splitting of VMAs seems problematic, because we don't duplicate/adjust the >> reservation when splitting the VMA. Instead, in memtype_erase() -- called >> during zapping/munmap -- we shrink a reservation in case only the end >> address matches: Assume we split a VMA into A and B, both would share a >> reservation until B is unmapped. > > Glorious. I really oppose us making radical changes to splitting logic to > suit this one x86-specific feature. > > Seems to me the change should be within PAT... Yeah ... > >> >> So when unmapping B, the reservation would be updated to cover only A. When >> unmapping A, we would properly remove the now-shrunk reservation. That >> scenario describes the mremap() shrinking (old_size > new_size), where >> we split + unmap B, and the untrack_pfn_clear() on the new VMA when >> is wrong. >> >> What if we manage to split a VM_PFNMAP VMA into A and B and unmap A >> first? It would be broken because we would never free the reservation. >> Likely, there are ways to trigger such a VMA split outside of mremap(). > > This must have been a problem that already existed, and this is really > quite plausible in reality, which makes me wonder, again, how much PAT is > actually used in the wild. Probably people use it all the time, but we only fix stuff that we hit in practice. Fun. :) > > I may be mistaken of course (happy to be corrected) and it's used very > heavily and somehow this scenario doesn't occur. > > We should definitely add a test for this anyway :) even if 'skip' for now > while we figure out how to fix it (_without egregiously impacting split > code_). > > This can't happen in mremap() in any case as we are unmapping which happens > to split then always remove the latter VMA. > > OK so to get back into the split logic, this is: > > shrink_vma() (in mm/mremap.c) > -> do_vmi_munmap() > -> do_vmi_align_munmap() > -> vms_gather_munmap_vmas() > -> __split_vma() > (later called from do_vmi_align_munmap()) > -> vms_complete_munmap_vmas() > -> vms_clear_ptes() > -> unmap_vmas() > -> unmap_single_vma() > -> untrack_pfn() > -> free_pfn_range() > -> memtype_free() > -> memtype_erase() > > As simple as that! ;) My head started spinning when I followed that callchain :D > > Makes sense. > >> >> Affecting other VMA duplication was not intended, vm_area_dup() being >> used outside of kernel/fork.c was an oversight. So let's fix that for; >> how to handle VMA splits better should be investigated separately. > > To reiterate, I think this should be handled within PAT itself, rather than > result in changes to VMA code, unless it results in us adding sensible > hooks there. Yes. I wish we could remove it; I'm afraid we can;t [...] > Anyway, having gone through your excellent descriptions (albeit, feeling > the same pain as you did :P), and looking at the logic, I agree this patch > is correct, so: > > Reviewed-by: Lorenzo Stoakes Thanks! > >> --- >> >> This VM_PAT code really wants me to scream at my computer. So far it didn't >> succeed, but I am close. Well, at least now I understand how it interacts >> with VMA splitting ... > > Well, I'm also quite scared of it, and indeed also relate ;) How heavily > used is PAT? We do seem to constantly run into problems with getting it to > behave itself wrt VMA code. > > We recently had to remove some quite egregious hooks in VMA code which was > a pain, is there a better way of doing this? > > I really do hate this 'randomly call a function in various spots and do > something specific for feature X' pattern that we use for hugetlb, uffd, > this, and other stuff. I hate it. Probably the right way of attaching such metadata to a VMA would be remembering it alongside the VMA in a very simple way. For example, when we perform a reservation we would allocate a refcounted object and assign it to the VMA (pointer, xarray, whatever). Duplicating the VMA would increase the refcount. Freeing a VMA would decrease the refcount. Once the refcount goes to zero, we undo the reservation and free the object. We would not adjust a reservation on partial VMA unmap (split + unmap A or B), but I strongly assume that would just be fine as long as we undo the reservation once the refcount goes to 0. There is one complication: this nasty memremap.c that abuses it for !vma handling. But that could be split out and handled differently. > > We need to use (carefully constrained!) generic hooks for this kind of > thing. > > At least tell me the mremap() refactor I did made this less horrible? ;) haha, absolutely :P -- Cheers, David / dhildenb