From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754282Ab2LNH17 (ORCPT ); Fri, 14 Dec 2012 02:27:59 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:52153 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752147Ab2LNH17 (ORCPT ); Fri, 14 Dec 2012 02:27:59 -0500 Date: Fri, 14 Dec 2012 07:27:55 +0000 From: Al Viro To: Andy Lutomirski Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Ingo Molnar , Michel Lespinasse , Hugh Dickins , J??rn Engel Subject: Re: [PATCH] mm: Downgrade mmap_sem before locking or populating on mmap Message-ID: <20121214072755.GR4939@ZenIV.linux.org.uk> References: <3b624af48f4ba4affd78466b73b6afe0e2f66549.1355463438.git.luto@amacapital.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3b624af48f4ba4affd78466b73b6afe0e2f66549.1355463438.git.luto@amacapital.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 13, 2012 at 09:49:43PM -0800, Andy Lutomirski wrote: > This is a serious cause of mmap_sem contention. MAP_POPULATE > and MCL_FUTURE, in particular, are disastrous in multithreaded programs. > > Signed-off-by: Andy Lutomirski > --- > > Sensible people use anonymous mappings. I write kernel patches :) > > I'm not entirely thrilled by the aesthetics of this patch. The MAP_POPULATE case > could also be improved by doing it without any lock at all. This is still a big > improvement, though. Wait a minute. get_user_pages() relies on ->mmap_sem being held. Unless I'm seriously misreading your patch it removes that protection. And yes, I'm aware of execve-related exception; it's in special circumstances - bprm->mm is guaranteed to be not shared (and we need to rearchitect that area anyway, but that's a separate story).