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 X-Spam-Level: X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BD560C433C1 for ; Tue, 30 Mar 2021 16:32:18 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 262E76198A for ; Tue, 30 Mar 2021 16:32:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 262E76198A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 98DB96B0081; Tue, 30 Mar 2021 12:32:17 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 93C6E6B0082; Tue, 30 Mar 2021 12:32:17 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 7E1276B0083; Tue, 30 Mar 2021 12:32:17 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0192.hostedemail.com [216.40.44.192]) by kanga.kvack.org (Postfix) with ESMTP id 5E7146B0081 for ; Tue, 30 Mar 2021 12:32:17 -0400 (EDT) Received: from smtpin12.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 199118249980 for ; Tue, 30 Mar 2021 16:32:17 +0000 (UTC) X-FDA: 77977083114.12.B776A72 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by imf20.hostedemail.com (Postfix) with ESMTP id D98D4130 for ; Tue, 30 Mar 2021 16:32:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617121924; 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; bh=5DxGbfaWBQohyHUg4T2hAugoPQ+5ugb6cQ8DhRFqjyc=; b=iDyNlZ/seVctelY43CCT4EPpSCK5tUoC/t8D5FWhZbD64YRtP8H9eA6DGIT/d1e1Bzxq7w /SEDn4InCr2Qu2vyAGkR8DnxrguXAdaQ+qWZEMK/NU4pi6H9y6besiiUa0fhniohllMUXN vpERlPyVgCisT5isl0gBb7MERA1dBfk= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-332-4doft93uOSKqpQWwdKTZlg-1; Tue, 30 Mar 2021 12:32:02 -0400 X-MC-Unique: 4doft93uOSKqpQWwdKTZlg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id F06F184BA40; Tue, 30 Mar 2021 16:31:58 +0000 (UTC) Received: from [10.36.114.210] (ovpn-114-210.ams2.redhat.com [10.36.114.210]) by smtp.corp.redhat.com (Postfix) with ESMTP id 12B9B10016DB; Tue, 30 Mar 2021 16:31:42 +0000 (UTC) Subject: Re: [PATCH v1 2/5] mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault/prealloc memory From: David Hildenbrand To: Jann Horn Cc: kernel list , Linux-MM , Andrew Morton , Arnd Bergmann , Michal Hocko , Oscar Salvador , Matthew Wilcox , Andrea Arcangeli , Minchan Kim , Jason Gunthorpe , Dave Hansen , Hugh Dickins , Rik van Riel , "Michael S . Tsirkin" , "Kirill A . Shutemov" , Vlastimil Babka , Richard Henderson , Ivan Kokshaysky , Matt Turner , Thomas Bogendoerfer , "James E.J. Bottomley" , Helge Deller , Chris Zankel , Max Filippov , Mike Kravetz , Peter Xu , Rolf Eike Beer , linux-alpha@vger.kernel.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-arch , Linux API References: <20210317110644.25343-1-david@redhat.com> <20210317110644.25343-3-david@redhat.com> <2bab28c7-08c0-7ff0-c70e-9bf94da05ce1@redhat.com> <26227fc6-3e7b-4e69-f69d-4dc2a67ecfe8@redhat.com> Organization: Red Hat GmbH Message-ID: <54165ffe-dbf7-377a-a710-d15be4701f20@redhat.com> Date: Tue, 30 Mar 2021 18:31:41 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <26227fc6-3e7b-4e69-f69d-4dc2a67ecfe8@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: D98D4130 X-Stat-Signature: f3fsgacfaqcg3319yeupr67dhucqcccb Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf20; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=170.10.133.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1617121931-847470 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: On 30.03.21 18:30, David Hildenbrand wrote: > On 30.03.21 18:21, Jann Horn wrote: >> On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand wrote: >>>>> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, >>>>> + unsigned long end, bool write, int *locked) >>>>> +{ >>>>> + struct mm_struct *mm = vma->vm_mm; >>>>> + unsigned long nr_pages = (end - start) / PAGE_SIZE; >>>>> + int gup_flags; >>>>> + >>>>> + VM_BUG_ON(!PAGE_ALIGNED(start)); >>>>> + VM_BUG_ON(!PAGE_ALIGNED(end)); >>>>> + VM_BUG_ON_VMA(start < vma->vm_start, vma); >>>>> + VM_BUG_ON_VMA(end > vma->vm_end, vma); >>>>> + mmap_assert_locked(mm); >>>>> + >>>>> + /* >>>>> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit >>>>> + * a poisoned page. >>>>> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. >>>>> + * !FOLL_FORCE: Require proper access permissions. >>>>> + */ >>>>> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; >>>>> + if (write) >>>>> + gup_flags |= FOLL_WRITE; >>>>> + >>>>> + /* >>>>> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings >>>>> + * or with insufficient permissions. >>>>> + */ >>>>> + return __get_user_pages(mm, start, nr_pages, gup_flags, >>>>> + NULL, NULL, locked); >>>> >>>> You mentioned in the commit message that you don't want to actually >>>> dirty all the file pages and force writeback; but doesn't >>>> POPULATE_WRITE still do exactly that? In follow_page_pte(), if >>>> FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: >>> >>> Well, I mention that POPULATE_READ explicitly doesn't do that. I >>> primarily set it because populate_vma_page_range() also sets it. >>> >>> Is it safe to *not* set it? IOW, fault something writable into a page >>> table (where the CPU could dirty it without additional page faults) >>> without marking it accessed? For me, this made logically sense. Thus I >>> also understood why populate_vma_page_range() set it. >> >> FOLL_TOUCH doesn't have anything to do with installing the PTE - it >> essentially means "the caller of get_user_pages wants to read/write >> the contents of the returned page, so please do the same things you >> would do if userspace was accessing the page". So in particular, if >> you look up a page via get_user_pages() with FOLL_WRITE|FOLL_TOUCH, >> that tells the MM subsystem "I will be writing into this page directly >> from the kernel, bypassing the userspace page tables, so please mark >> it as dirty now so that it will be properly written back later". Part >> of that is that it marks the page as recently used, which has an >> effect on LRU pageout behavior, I think - as far as I understand, that >> is why populate_vma_page_range() uses FOLL_TOUCH. >> >> If you look at __get_user_pages(), you can see that it is split up >> into two major parts: faultin_page() for creating PTEs, and >> follow_page_mask() for grabbing pages from PTEs. faultin_page() >> ignores FOLL_TOUCH completely; only follow_page_mask() uses it. >> >> In a way I guess maybe you do want the "mark as recently accessed" >> part that FOLL_TOUCH would give you without FOLL_WRITE? But I think >> you very much don't want the dirtying that FOLL_TOUCH|FOLL_WRITE leads >> to. Maybe the ideal approach would be to add a new FOLL flag to say "I >> only want to mark as recently used, I don't want to dirty". Or maybe >> it's enough to just leave out the FOLL_TOUCH entirely, I don't know. > > Any thoughts why populate_vma_page_range() does it? Sorry, I missed the explanation above - thanks! -- Thanks, David / dhildenb