From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752561AbdAYTW4 (ORCPT ); Wed, 25 Jan 2017 14:22:56 -0500 Received: from gum.cmpxchg.org ([85.214.110.215]:39852 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752302AbdAYTWz (ORCPT ); Wed, 25 Jan 2017 14:22:55 -0500 Date: Wed, 25 Jan 2017 14:22:45 -0500 From: Johannes Weiner To: Michal Hocko Cc: Tetsuo Handa , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v6] mm: Add memory allocation watchdog kernel thread. Message-ID: <20170125192245.GA19321@cmpxchg.org> References: <1478416501-10104-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20170125181150.GA16398@cmpxchg.org> <20170125184548.GB32041@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170125184548.GB32041@dhcp22.suse.cz> User-Agent: Mutt/1.7.2 (2016-11-26) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 25, 2017 at 07:45:49PM +0100, Michal Hocko wrote: > On Wed 25-01-17 13:11:50, Johannes Weiner wrote: > [...] > > >From 6420cae52cac8167bd5fb19f45feed2d540bc11d Mon Sep 17 00:00:00 2001 > > From: Johannes Weiner > > Date: Wed, 25 Jan 2017 12:57:20 -0500 > > Subject: [PATCH] mm: page_alloc: __GFP_NOWARN shouldn't suppress stall > > warnings > > > > __GFP_NOWARN, which is usually added to avoid warnings from callsites > > that expect to fail and have fallbacks, currently also suppresses > > allocation stall warnings. These trigger when an allocation is stuck > > inside the allocator for 10 seconds or longer. > > > > But there is no class of allocations that can get legitimately stuck > > in the allocator for this long. This always indicates a problem. > > > > Always emit stall warnings. Restrict __GFP_NOWARN to alloc failures. > > Tetsuo has already suggested something like this and I didn't really > like it because it makes the semantic of the flag confusing. The mask > says to not warn while the kernel log might contain an allocation splat. > You are right that stalling for 10s seconds means a problem on its own > but on the other hand I can imagine somebody might really want to have > clean logs and the last thing we want is to have another gfp flag for > that purpose. I don't think it's confusing. __GFP_NOWARN tells the allocator whether an allocation failure can be handled or whether it constitutes a bug. If we agree that stalling for 10s is a bug, then we should emit the warnings. Tying this to whether the caller can handle an allocation failure is non-sensical. Not warning about a bug because the user would prefer clean logs is... somewhat out there.