From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f70.google.com (mail-wm0-f70.google.com [74.125.82.70]) by kanga.kvack.org (Postfix) with ESMTP id AE7656B0033 for ; Mon, 16 Oct 2017 03:38:38 -0400 (EDT) Received: by mail-wm0-f70.google.com with SMTP id q203so13428716wmb.0 for ; Mon, 16 Oct 2017 00:38:38 -0700 (PDT) Received: from newverein.lst.de (verein.lst.de. [213.95.11.211]) by mx.google.com with ESMTPS id v2si5232597wra.381.2017.10.16.00.38.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 16 Oct 2017 00:38:37 -0700 (PDT) Date: Mon, 16 Oct 2017 09:38:36 +0200 From: Christoph Hellwig Subject: Re: [PATCH v9 1/6] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags Message-ID: <20171016073836.GB28778@lst.de> References: <150776922692.9144.16963640112710410217.stgit@dwillia2-desk3.amr.corp.intel.com> <150776923320.9144.6119113178052262946.stgit@dwillia2-desk3.amr.corp.intel.com> <20171012135127.GG29293@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Linus Torvalds Cc: Jan Kara , Dan Williams , "linux-nvdimm@lists.01.org" , Arnd Bergmann , Linux API , linux-xfs , linux-mm , Andy Lutomirski , linux-fsdevel , Andrew Morton , Christoph Hellwig On Thu, Oct 12, 2017 at 09:32:17AM -0700, Linus Torvalds wrote: > On Thu, Oct 12, 2017 at 6:51 AM, Jan Kara wrote: > > > > When thinking a bit more about this I've realized one problem: Currently > > user can call mmap() with MAP_SHARED type and MAP_SYNC or MAP_DIRECT flags > > and he will get the new semantics (if the kernel happens to support it). I > > think that is undesirable [..] > > Why? > > If you have a performance preference for MAP_DIRECT or something like > that, but you don't want to *enforce* it, you'd use just plain > MAP_SHARED with it. > > Ie there may well be "I want this to work, possibly with downsides" issues. > > So it seems to be a reasonable model, and disallowing it seems to > limit people and not really help anything. I don't think for MAP_DIRECT it matters (and I think we shouldn't have MAP_DIRECT to start with, see the discussions later in the thread). But for the main use case, MAP_SYNC you really want a hard error when you don't get it. And while we could tell people that they should only use MAP_SYNC with MAP_SHARED_VALIDATE instead of MAP_SHARED chances that they get it wrong are extremely high. On the other hand if you really only want a flag to optimize calling mmap twice is very little overhead, and a very good documentation of you intent: addr = mmap(...., MAP_SHARED_VALIDATE | MAP_DIRECT, ...); if (!addr && errno = EOPNOTSUPP) { /* MAP_DIRECT didn't work, we'll just cope using blah, blah */ addr = mmap(...., MAP_SHARED, ...); } if (!addr) goto handle_error; -- 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