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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 827B9C433DB for ; Mon, 22 Feb 2021 13:00:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3A561600CF for ; Mon, 22 Feb 2021 13:00:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231707AbhBVNAJ (ORCPT ); Mon, 22 Feb 2021 08:00:09 -0500 Received: from mx2.suse.de ([195.135.220.15]:49322 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231685AbhBVM7L (ORCPT ); Mon, 22 Feb 2021 07:59:11 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1613998687; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=VapX0P1cjmfTsM5vWm6XJfMkWbyfERrb6gAU3Sl6W4Q=; b=WJkXS1MRD8pmy72Tqm8tbLj4R9dTBxozMzQZtygXtuV4ch1Z4xqZsV9zWtePWU+3bU2bTa o+K5m3839jIF7PigP8a7p2FBcw0YyQiTe/A055r3eVYeXRHhAJ9DL6F0hly9nbPqxMVTqo ZD1DSkJ2z7GHZJcsJHgnMOzqIX3Awrc= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 1D631AE1C; Mon, 22 Feb 2021 12:58:07 +0000 (UTC) Date: Mon, 22 Feb 2021 13:56:35 +0100 From: Michal Hocko To: David Hildenbrand 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 Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory Message-ID: References: <20210217154844.12392-1-david@redhat.com> <640738b5-a47e-448b-586d-a1fb80131891@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <640738b5-a47e-448b-586d-a1fb80131891@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On Sat 20-02-21 10:12:26, David Hildenbrand wrote: [...] > Thinking about MADV_POPULATE vs. MADV_POPULATE_WRITE I wonder if it would be > more versatile to break with existing MAP_POPULATE semantics and directly go > with > > MADV_POPULATE_READ: simulate user space read access without actually > reading. Trigger a read fault if required. > > MADV_POPULATE_WRITE: simulate user space write access without actually > writing. Trigger a write fault if required. > > For my use case, I could use MADV_POPULATE_WRITE on anonymous memory and > RAM-backed files (shmem/hugetlb) - I would not have a minor fault when the > guest inside the VM first initializes memory. This mimics how QEMU currently > preallocates memory. > > However, I would use MADV_POPULATE_READ on any !RAM-backed files where we > actually have to write-back to a (slow?) device. Dirtying everything > although the guest might not actually consume it in the near future might be > undesired. Isn't what the current mm_populate does? if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE) gup_flags |= FOLL_WRITE; So it will write fault to shared memory mappings but it will touch others. -- Michal Hocko SUSE Labs