From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750875AbbC2IC3 (ORCPT ); Sun, 29 Mar 2015 04:02:29 -0400 Received: from mail-wi0-f169.google.com ([209.85.212.169]:34573 "EHLO mail-wi0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751024AbbC2ICW (ORCPT ); Sun, 29 Mar 2015 04:02:22 -0400 Message-ID: <5517B18A.3050305@plexistor.com> Date: Sun, 29 Mar 2015 11:02:18 +0300 From: Boaz Harrosh User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 MIME-Version: 1.0 To: Dave Chinner , Matthew Wilcox CC: Matthew Wilcox , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, msharbiani@twopensource.com Subject: Re: Should implementations of ->direct_access be allowed to sleep? References: <1411677218-29146-1-git-send-email-matthew.r.wilcox@intel.com> <1411677218-29146-22-git-send-email-matthew.r.wilcox@intel.com> <20150324185046.GA4994@whiteoak.sf.office.twttr.net> <20150326170918.GO4003@linux.intel.com> <20150326193224.GA28129@dastard> In-Reply-To: <20150326193224.GA28129@dastard> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/26/2015 09:32 PM, Dave Chinner wrote: <> >> I'm leaning towards the latter. But I'm not sure what GFP flags to >> recommend that brd use ... GFP_NOWAIT | __GFP_ZERO, perhaps? > > What, so we get random IO failures under memory pressure? > > I really think we should allow .direct_access to sleep. It means we > can use existing drivers and it also allows future implementations > that might require, say, RDMA to be performed to update a page > before access is granted. i.e. .direct_access is the first hook into > the persistent device at page fault time.... > I agree with Dave. Last I tried (couple years ago) doing any allocation GFP_NOWAIT on FS IO paths fails really badly in all kind of surprising ways. The Kernel is built in to that allocation pressure. I think that ->direct_access should not be any different then any other block-device access, ie allow to sleep. With brd a user can make sure not to sleep if he pre-allocates ie call ->direct_access at least once on a given offset-length. But I would not like to even do that guaranty. ->direct_access should be allowed to sleep. Well written code has many ways to allow sleep yet be very low latency. (So I do not see what we are missing) > Cheers, > Dave. Thanks Boaz