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 B6BD0C4345F for ; Thu, 2 May 2024 14:33:48 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 393C66B0087; Thu, 2 May 2024 10:33:48 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 342C16B0089; Thu, 2 May 2024 10:33:48 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 1E50A6B008A; Thu, 2 May 2024 10:33:48 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id F0FBA6B0087 for ; Thu, 2 May 2024 10:33:47 -0400 (EDT) Received: from smtpin19.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 8EB65A0E4A for ; Thu, 2 May 2024 14:33:47 +0000 (UTC) X-FDA: 82073699694.19.817C6A9 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf26.hostedemail.com (Postfix) with ESMTP id 3674E14000B for ; Thu, 2 May 2024 14:33:45 +0000 (UTC) Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=dLLIh3Lp; spf=pass (imf26.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1714660425; 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=CHYxkIQ8ku9Et4Lmsh2/94NqfW9qDb54vDaZA3KSs4s=; b=Lar1+bSyCL49bLy7k9jYSLs100z8tvqKG+P+ZMjtkCxNHY8DT+T2G77gQNHLqrCcf1akz8 5iw5mqYlEECby1cru4EGJOvNydBHkuMzKiWn00gm2DG048FIruu+qo/N6+cZymHvHWuPtE RDeqx0FAcI6hx4ovhdI6nsap6X1P3aE= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1714660425; a=rsa-sha256; cv=none; b=p4gjMrHaHgB/cLTE5YriP3G/xQp5vC/ZaldqGDQWIMgHyYO3Ht8SpAyFxvM+XRUaqdpH3C 8VtTnZqi322SujClkxg4UiI1VkC9E0p7Y/DPWOOcElkfzCQGVJ7fMvvWheye5JO8lKiney DcnWPN80nmv/p6Bhv9Ql0sRsuPJO5ck= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=dLLIh3Lp; spf=pass (imf26.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1714660424; 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=CHYxkIQ8ku9Et4Lmsh2/94NqfW9qDb54vDaZA3KSs4s=; b=dLLIh3LpFvAGHRus/7rHX1LiSFyKWxPtGXR/2+SlYOxiSCwdd/HKDwbiok/rW0pvRzZNHn Ho8ZD+DCNkULsm5FKQnsyy54J0A3XUhpKuGi5ZmKVokiLjZo6r5LmniIVOHRjhHLWemljx BN8qqMMbARsbeU61Ldu9pCRHUnUk0jA= Received: from mail-wr1-f70.google.com (mail-wr1-f70.google.com [209.85.221.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-451-IS-WB7IOOlOkp0o3XhrcFg-1; Thu, 02 May 2024 10:33:42 -0400 X-MC-Unique: IS-WB7IOOlOkp0o3XhrcFg-1 Received: by mail-wr1-f70.google.com with SMTP id ffacd0b85a97d-34dc7b83209so1335980f8f.2 for ; Thu, 02 May 2024 07:33:42 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1714660422; x=1715265222; h=content-transfer-encoding:in-reply-to:organization:autocrypt:from :content-language: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=CHYxkIQ8ku9Et4Lmsh2/94NqfW9qDb54vDaZA3KSs4s=; b=beKU2abXx7k8zAEC5Mu872YtJXwNeQX8j5zGbSJUh/J7ODTO3nu6K9IBtbmnYd5Z1G khBLyrE8ECz6MMOafYl6UZlGb2w04K0vJUA1J6iiErUG1LWw52PN3M7BR1y6LPtXV4UG Jekoc8dO5E4sMeZ3ngsvJBscd6COgJNBUSgx6H6WPq2WuLMuwyQRpD7VoDOncIpvF+D4 O6RiAvnrB6SfJHSFmFw59poOVwc1OYOzwp0PlJ92EpR1hb6ySOB2CyiCtDs0r4dCXoG6 SJ2IYdbOQJdXBAfXEr7OPtw/iJczXbLIvmfZkxs77/xkNccnJ0TO++cXjbuAaqp/vE5x Fkqw== X-Forwarded-Encrypted: i=1; AJvYcCXmr7iH/jxLoyd9xbEMLxE7X3fhh+Tc4j14X4HI8yU02YP9XXWhC5FwfydPz+NrYF7df1rl7c3RXDmPu1t7MWfdjsw= X-Gm-Message-State: AOJu0Yw7tb1l83enGVni7rMyPgqfbp68sDV1m8IyV31OBvFUzSHl+oYf LM6g9YLNEpeaDVP75o2TmgTXqNIem7Jiu8MMrzAvwgnvuCM6op2mlALFraYAmU2NzISxjIkXTNm WjUu47NOr2QzlnBPYUosbgZ5MQzadx1BBjbKd24/emfUz9anl0pja68t+ X-Received: by 2002:adf:e0c1:0:b0:34b:8bf:5fa5 with SMTP id m1-20020adfe0c1000000b0034b08bf5fa5mr4151wri.7.1714660421736; Thu, 02 May 2024 07:33:41 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHSVc3GjlITyXVsLJLEkrUivUde0tR0KHggikIou3sLGcFYSBzjyDOHqUqvmqNv9AnT4rQVsg== X-Received: by 2002:adf:e0c1:0:b0:34b:8bf:5fa5 with SMTP id m1-20020adfe0c1000000b0034b08bf5fa5mr4127wri.7.1714660421195; Thu, 02 May 2024 07:33:41 -0700 (PDT) Received: from ?IPV6:2003:cb:c71e:bf00:eba1:3ab9:ab0f:d676? (p200300cbc71ebf00eba13ab9ab0fd676.dip0.t-ipconnect.de. [2003:cb:c71e:bf00:eba1:3ab9:ab0f:d676]) by smtp.gmail.com with ESMTPSA id dy1-20020a0560000e4100b0034afaa9ca47sm1411938wrb.1.2024.05.02.07.33.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 02 May 2024 07:33:40 -0700 (PDT) Message-ID: <9e9ac4a3-461f-4ada-a41c-4432c947c818@redhat.com> Date: Thu, 2 May 2024 16:33:39 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/2] mm/hugetlb: document why hugetlb uses folio_mapcount() for COW reuse decisions To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Andrew Morton , Muchun Song , Shuah Khan References: <20240502085259.103784-1-david@redhat.com> <20240502085259.103784-3-david@redhat.com> 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: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: 38qfaceou7qgwux8gpkmx4dg9piau3ca X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 3674E14000B X-HE-Tag: 1714660425-141557 X-HE-Meta: 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 xRN1R9Lb 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000015, 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 02.05.24 16:28, Peter Xu wrote: > On Thu, May 02, 2024 at 10:52:59AM +0200, David Hildenbrand wrote: >> Let's document why hugetlb still uses folio_mapcount() and is prone to >> leaking memory between processes, for example using vmsplice() that >> still uses FOLL_GET. >> >> More details can be found in [1], especially around how hugetlb pages >> cannot really be overcommitted, and why we don't particularly care about >> these vmsplice() leaks for hugetlb -- in contrast to ordinary memory. >> >> [1] https://lore.kernel.org/all/8b42a24d-caf0-46ef-9e15-0f88d47d2f21@redhat.com/ >> >> Suggested-by: Peter Xu >> Signed-off-by: David Hildenbrand >> --- >> mm/hugetlb.c | 7 +++++++ >> 1 file changed, 7 insertions(+) >> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >> index 417fc5cdb6eeb..a7efb350f5d07 100644 >> --- a/mm/hugetlb.c >> +++ b/mm/hugetlb.c >> @@ -5963,6 +5963,13 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio, >> /* >> * If no-one else is actually using this page, we're the exclusive >> * owner and can reuse this page. >> + * >> + * Note that we don't rely on the (safer) folio refcount here, because >> + * copying the hugetlb folio when there are unexpected (temporary) >> + * folio references could harm simple fork()+exit() users when >> + * we run out of free hugetlb folios: we would have to kill processes >> + * in scenarios that used to work. As a side effect, there can still >> + * be leaks between processes, for example, with FOLL_GET users. >> */ >> if (folio_mapcount(old_folio) == 1 && folio_test_anon(old_folio)) { >> if (!PageAnonExclusive(&old_folio->page)) { > > Thanks for preparing such updates, David. > > However is fork+exit the real problem? E.g. if a child simply fork, do > something, then exit, I don't see a major issue even if we follow refcount > here (despite the "check against 1 or 2 or 3" issue, where hugetlb_fault > can take one already). As long as the child quits, all ref / map counts > will be released then. If the child needs to write to ANON|PRIV it needs > to manage hugetlb reservations anyways. The PAE flag was cleared and if there is any unexpected (temporary) reference (page migration, lockless swapcache lookups, whatever), we're in trouble. > > In the case of vmsplice it's kind of malicious, and holding that refcount > (with 0 mapcount) doesn't sound the common scenario to me. Yes, I'm not that concerned about something that that APP would be triggering (vmsplice). > > IIUC if we need to keep this, it was more about the case where (as you > correctly mentioned in another follow up reply) hugetlb isn't that flexible > to memory overcommits, and in many cases it won't have extra pages floating > around to allow adhoc CoWs? While random refcount boost is easy to happen, > and here the problem is we simply cannot identify that v.s. vmsplice() > malicious takers. Yes, exactly. -- Cheers, David / dhildenb