Return-Path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:48454 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754320AbeDYPZM (ORCPT ); Wed, 25 Apr 2018 11:25:12 -0400 Date: Wed, 25 Apr 2018 11:25:09 -0400 (EDT) From: Mikulas Patocka To: Michal Hocko cc: Richard Weinberger , LKML , Artem Bityutskiy , David Woodhouse , Brian Norris , Boris Brezillon , Marek Vasut , Cyrille Pitchen , "Theodore Ts'o" , Andreas Dilger , Steven Whitehouse , Bob Peterson , Trond Myklebust , Anna Schumaker , Adrian Hunter , Philippe Ombredanne , Kate Stewart , linux-mtd@lists.infradead.org, linux-ext4@vger.kernel.org, cluster-devel@redhat.com, linux-nfs@vger.kernel.org, linux-mm@kvack.org Subject: Re: vmalloc with GFP_NOFS In-Reply-To: <20180425144557.GD17484@dhcp22.suse.cz> Message-ID: References: <20180424162712.GL17484@dhcp22.suse.cz> <3732370.1623zxSvNg@blindfold> <20180424192803.GT17484@dhcp22.suse.cz> <3894056.cxOY6eVYVp@blindfold> <20180424230943.GY17484@dhcp22.suse.cz> <20180424232517.GC17484@dhcp22.suse.cz> <20180425144557.GD17484@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-nfs-owner@vger.kernel.org List-ID: On Wed, 25 Apr 2018, Michal Hocko wrote: > On Wed 25-04-18 08:43:32, Mikulas Patocka wrote: > > > > > > On Tue, 24 Apr 2018, Michal Hocko wrote: > > > > > On Tue 24-04-18 19:17:12, Mikulas Patocka wrote: > > > > > > > > > > > > On Tue, 24 Apr 2018, Michal Hocko wrote: > > > > > > > > > > So in a perfect world a filesystem calls memalloc_nofs_save/restore and > > > > > > always uses GFP_KERNEL for kmalloc/vmalloc? > > > > > > > > > > Exactly! And in a dream world those memalloc_nofs_save act as a > > > > > documentation of the reclaim recursion documentation ;) > > > > > -- > > > > > Michal Hocko > > > > > SUSE Labs > > > > > > > > BTW. should memalloc_nofs_save and memalloc_noio_save be merged into just > > > > one that prevents both I/O and FS recursion? > > > > > > Why should FS usage stop IO altogether? > > > > Because the IO may reach loop and loop may redirect it to the same > > filesystem that is running under memalloc_nofs_save and deadlock. > > So what is the difference with the current GFP_NOFS? My point is that filesystems should use GFP_NOIO too. If alloc_pages(GFP_NOFS) issues some random I/O to some block device, the I/O may be end up being redirected (via block loop device) to the filesystem that is calling alloc_pages(GFP_NOFS). > > > > memalloc_nofs_save allows submitting bios to I/O stack and the bios > > > > created under memalloc_nofs_save could be sent to the loop device and the > > > > loop device calls the filesystem... > > > > > > Don't those use NOIO context? > > > > What do you mean? > > That the loop driver should make sure it will not recurse. The scope API > doesn't add anything new here. The loop driver doesn't recurse. The loop driver will add the request to a queue and wake up a thread that processes it. But if the request queue is full, __get_request will wait until the loop thread finishes processing some other request. It doesn't recurse, but it waits until the filesystem makes some progress. Mikulas