From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx155.postini.com [74.125.245.155]) by kanga.kvack.org (Postfix) with SMTP id 5DEB56B0044 for ; Fri, 14 Dec 2012 06:15:11 -0500 (EST) Received: by mail-vb0-f41.google.com with SMTP id l22so3844644vbn.14 for ; Fri, 14 Dec 2012 03:15:10 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20121214072755.GR4939@ZenIV.linux.org.uk> References: <3b624af48f4ba4affd78466b73b6afe0e2f66549.1355463438.git.luto@amacapital.net> <20121214072755.GR4939@ZenIV.linux.org.uk> From: Andy Lutomirski Date: Fri, 14 Dec 2012 03:14:50 -0800 Message-ID: Subject: Re: [PATCH] mm: Downgrade mmap_sem before locking or populating on mmap Content-Type: text/plain; charset=ISO-8859-1 Sender: owner-linux-mm@kvack.org List-ID: To: Al Viro Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Ingo Molnar , Michel Lespinasse , Hugh Dickins , J??rn Engel On Thu, Dec 13, 2012 at 11:27 PM, Al Viro wrote: > 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). Unless I completely screwed up the patch, ->mmap_sem is still held for read (it's downgraded from write). It's just not held for write anymore. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org