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 BECE7C4320A for ; Mon, 16 Aug 2021 14:43:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A64896054E for ; Mon, 16 Aug 2021 14:43:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232912AbhHPOn5 (ORCPT ); Mon, 16 Aug 2021 10:43:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39354 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232281AbhHPOnn (ORCPT ); Mon, 16 Aug 2021 10:43:43 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4F38CC0613C1 for ; Mon, 16 Aug 2021 07:42:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=WEkk9bTRSPeIztpf1WOtGTbt+rB7VbtYlTiQl5in/5o=; b=GN4Fvc2ClJf2hxkpESl7npbmyY 7UDRJDgS9s6lwS74oxsT2qgbCmZXrAUTPn1G61FqkgE4dW8DPc3gwE0UWVJRJpEVIrPuMa+LJYKUc 3DCXx4TUysvoLi/Eg/KuVwjCPO+kJDZFPbLd4nQqbH4gj/vdp5ULf3l2gV6x9PZrRqupSSyWt5QUg cUTXuKaPQD4/oJHEK+0mpbzevDfxhcrAWJgN3QxqNjO5mLG4jDHs9vL9LU7tpQPT6DNWpu5s9Si+3 kWBFaCER/CYTWba7I59U0s5ELs2gVWCAsOzSOCqpT9kl+/SEhYwxN14GJTjsdt0mzaCeILN7F/l0K SU25MxsQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1mFdnM-001UOc-NM; Mon, 16 Aug 2021 14:41:02 +0000 Date: Mon, 16 Aug 2021 15:40:56 +0100 From: Matthew Wilcox To: David Hildenbrand Cc: Khalid Aziz , "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" , Steven Sistare , Anthony Yznaga , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "Gonglei (Arei)" Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC Message-ID: References: <55720e1b39cff0a0f882d8610e7906dc80ea0a01.camel@oracle.com> <88884f55-4991-11a9-d330-5d1ed9d5e688@redhat.com> <40bad572-501d-e4cf-80e3-9a8daa98dc7e@redhat.com> <3ce1f52f-d84d-49ba-c027-058266e16d81@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3ce1f52f-d84d-49ba-c027-058266e16d81@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 16, 2021 at 04:33:09PM +0200, David Hildenbrand wrote: > > > I did not follow why we have to play games with MAP_PRIVATE, and having > > > private anonymous pages shared between processes that don't COW, introducing > > > new syscalls etc. > > > > It's not about SHMEM, it's about file-backed pages on regular > > filesystems. I don't want to have XFS, ext4 and btrfs all with their > > own implementations of ARCH_WANT_HUGE_PMD_SHARE. > > Let me ask this way: why do we have to play such games with MAP_PRIVATE? Are you referring to this? : Mappings within this address range behave as if they were shared : between threads, so a write to a MAP_PRIVATE mapping will create a : page which is shared between all the sharers. If so, that's a misunderstanding, because there are no games being played. What Khalid's saying there is that because the page tables are already shared for that range of address space, the COW of a MAP_PRIVATE will create a new page, but that page will be shared between all the sharers. The second write to a MAP_PRIVATE page (by any of the sharers) will not create a COW situation. Just like if all the sharers were threads of the same process.