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 4FF9CC43217 for ; Thu, 18 Feb 2021 12:19:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1BC2864E2F for ; Thu, 18 Feb 2021 12:19:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232585AbhBRMSM (ORCPT ); Thu, 18 Feb 2021 07:18:12 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:22421 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232032AbhBRLkP (ORCPT ); Thu, 18 Feb 2021 06:40:15 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1613648326; 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=3A4zaSGP/jDU0jQya7nJb+PtO132rNXYV3X5vurWHRM=; b=FsnxXWc2e03gNM9vf6/ZY/Yo7E9exlCDot+KyryDAdQ2JH3jarJj+EpjyHE1+DvUCQVup8 ruVnGUhFmzEx1y/JrlRJ9ko6K/iUOsnq5hh/NMrucNOPrVVyjvSihc7lQZVxqot01/fVmd 8hLWbHxsl5xzjkam9TZ7EQaVOA5WyIk= 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-280-fC5elPu5OAqWLO01C1hoPg-1; Thu, 18 Feb 2021 06:38:42 -0500 X-MC-Unique: fC5elPu5OAqWLO01C1hoPg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 0A5F3801965; Thu, 18 Feb 2021 11:38:39 +0000 (UTC) Received: from [10.36.114.59] (ovpn-114-59.ams2.redhat.com [10.36.114.59]) by smtp.corp.redhat.com (Postfix) with ESMTP id B12B16F7EC; Thu, 18 Feb 2021 11:38:28 +0000 (UTC) To: Michal Hocko Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Arnd Bergmann , Oscar Salvador , Matthew Wilcox , Andrea Arcangeli , Minchan Kim , Jann Horn , 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 , linux-alpha@vger.kernel.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org References: <20210217154844.12392-1-david@redhat.com> <3763a505-02d6-5efe-a9f5-40381acfbdfd@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory Message-ID: <740cdd51-137b-2b08-8b7f-9757d8d847cb@redhat.com> Date: Thu, 18 Feb 2021 12:38:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-parisc@vger.kernel.org >>>> If we hit >>>> hardware errors on pages, ignore them - nothing we really can or >>>> should do. >>>> 3. On errors during MADV_POPULATED, some memory might have been >>>> populated. Callers have to clean up if they care. >>> >>> How does caller find out? madvise reports 0 on success so how do you >>> find out how much has been populated? >> >> If there is an error, something might have been populated. In my QEMU >> implementation, I simply discard the range again, good enough. I don't think >> we need to really indicate "error and populated" or "error and not >> populated". > > Agreed. The wording just suggests that the syscall actually provides any > means for an effective way to handle those errors. Maybe you should just > stick with the first sentence and drop the second. Makes sense. "On errors during MADV_POPULATE, some memory might have been populated." > >>>> 4. Concurrent changes to the virtual memory layour are tolerated - we >>>> process each and every PFN only once, though. >>> >>> I do not understand this. madvise is about virtual address space not a >>> physical address space. >> >> What I wanted to express: if we detect a change in the mapping we don't >> restart at the beginning, we always make forward progress. We process each >> virtual address once (on a per-page basis, thus I accidentally used "PFN"). > > This is an implicit assumption. Your range can have the same page mapped > several times in the given address range and all you care about is that > you fault those which are not present during the virtual address space > walk. Your syscall can return and large part of the address space might > be unpopulated because memory reclaim just dropped those pages and that > would be fine. This shouldn't really imply memory presence - mlock does > that. "Concurrent changes to the virtual memory layout are tolerated. The range is processed exactly once." > >>>> 5. If MADV_POPULATE succeeds, all memory in the range can be accessed >>>> without SIGBUS. (of course, not if user space changed mappings in the >>>> meantime or KSM kicked in on anonymous memory). >>> >>> I do not see how KSM would change anything here and maybe it is not >>> really important to mention it. KSM should be really transparent from >>> the users space POV. Parallel and destructive virtual address space >>> operations are also expected to change the outcome and there is nothing >>> kernel do about at and provide any meaningful guarantees. I guess we >>> want to assume a reasonable userspace behavior here. >> >> It's just a note that we cannot protect from someone interfering >> (discard/ksm/whatever). I'm making that clearer in the cover letter. > > Again that is implicit expectation. madvise will not work for anybody > shooting an own foot. Okay, I'll drop that part, thanks! -- Thanks, David / dhildenb