From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759391AbdEVMJv (ORCPT ); Mon, 22 May 2017 08:09:51 -0400 Received: from mx2.suse.de ([195.135.220.15]:49032 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753008AbdEVMJo (ORCPT ); Mon, 22 May 2017 08:09:44 -0400 Date: Mon, 22 May 2017 14:09:37 +0200 From: Michal Hocko To: Mikulas Patocka Cc: Junaid Shahid , David Rientjes , Alasdair Kergon , Mike Snitzer , Andrew Morton , linux-mm@kvack.org, andreslc@google.com, gthelen@google.com, vbabka@suse.cz, linux-kernel@vger.kernel.org Subject: Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Message-ID: <20170522120937.GI8509@dhcp22.suse.cz> References: <20170518185040.108293-1-junaids@google.com> <20170518190406.GB2330@dhcp22.suse.cz> <1508444.i5EqlA1upv@js-desktop.svl.corp.google.com> <20170519074647.GC13041@dhcp22.suse.cz> <20170522093725.GF8509@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 22-05-17 08:00:11, Mikulas Patocka wrote: > > > On Mon, 22 May 2017, Michal Hocko wrote: > > > On Fri 19-05-17 19:43:23, Mikulas Patocka wrote: > > > > > > > > > On Fri, 19 May 2017, Michal Hocko wrote: > > > > > > > On Thu 18-05-17 19:50:46, Junaid Shahid wrote: > > > > > (Adding back the correct linux-mm email address and also adding linux-kernel.) > > > > > > > > > > On Thursday, May 18, 2017 01:41:33 PM David Rientjes wrote: > > > > [...] > > > > > > Let's ask Mikulas, who changed this from PF_MEMALLOC to __GFP_HIGH, > > > > > > assuming there was a reason to do it in the first place in two different > > > > > > ways. > > > > > > > > Hmm, the old PF_MEMALLOC used to have the following comment > > > > /* > > > > * Trying to avoid low memory issues when a device is > > > > * suspended. > > > > */ > > > > > > > > I am not really sure what that means but __GFP_HIGH certainly have a > > > > different semantic than PF_MEMALLOC. The later grants the full access to > > > > the memory reserves while the prior on partial access. If this is _really_ > > > > needed then it deserves a comment explaining why. > > > > -- > > > > Michal Hocko > > > > SUSE Labs > > > > > > Sometimes, I/O to a device mapper device is blocked until the userspace > > > daemon dmeventd does some action (for example, when dm-mirror leg fails, > > > dmeventd needs to mark the leg as failed in the lvm metadata and then > > > reload the device). > > > > > > The dmeventd daemon mlocks itself in memory so that it doesn't generate > > > any I/O. But it must be able to call ioctls. __GFP_HIGH is there so that > > > the ioctls issued by dmeventd have higher chance of succeeding if some I/O > > > is blocked, waiting for dmeventd action. It reduces the possibility of > > > low-memory-deadlock, though it doesn't eliminate it entirely. > > > > So what happens if the memory reserves are depleted. Do we deadlock? > > Yes, it will deadlock. That would be more than unfortunate and begs for a different solution. The thing is that __GFP_HIGH is not propagated to all allocations in the vmalloc proper. E.g. page table allocations are hardcoded GFP_KERNEL. > > Why is OOM killer insufficient to allow the further progress? > > I don't know if the OOM killer will or won't be triggered in this > situation, it depends on the people who wrote the OOM killer. I am not sure I understand. OOM killer is invoked for _all_ allocations <= PAGE_ALLOC_COSTLY_ORDER that do not have __GFP_NORETRY as long as the OOM killer is not disabled (oom_killer_disable) and that only happens from the PM suspend path which makes sure that no userspace is active at the time. AFAIU this is a userspace triggered path and so the later shouldn't apply to it and GFP_KERNEL should be therefore sufficient. Relying to a portion of memory reserves to prevent from deadlock seems fundamentaly broken to me. -- Michal Hocko SUSE Labs