From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2DC28C3A59C for ; Thu, 15 Aug 2019 13:21:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0548D2083B for ; Thu, 15 Aug 2019 13:21:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1565875292; bh=OmL7ZiiVEWZYmjPRvsn4aOS9NpnbHcdin1rQu8l8A+0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=KFIvHwiQcHnC/l16AbzH7CSGc7zNh5nsu7i76oq6yk1gLkILU7XZVePYo+xF8Ozbb cDQBCJI/7MvR1QE95tp5AbOxJAvsxapGjeGur465kOjEeBFOurDj4Ln0BU5uDuJO1h juHJVrbUYnlHmWIiGwGU6Txz41lUZZXrdpxa0FYE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732249AbfHONVb (ORCPT ); Thu, 15 Aug 2019 09:21:31 -0400 Received: from mx2.suse.de ([195.135.220.15]:42698 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1731822AbfHONVa (ORCPT ); Thu, 15 Aug 2019 09:21:30 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 6BF77AF2D; Thu, 15 Aug 2019 13:21:28 +0000 (UTC) Date: Thu, 15 Aug 2019 15:21:27 +0200 From: Michal Hocko To: Jason Gunthorpe Cc: LKML , linux-mm@kvack.org, DRI Development , Intel Graphics Development , Peter Zijlstra , Ingo Molnar , Andrew Morton , David Rientjes , Christian =?iso-8859-1?Q?K=F6nig?= , =?iso-8859-1?B?Suly9G1l?= Glisse , Masahiro Yamada , Wei Wang , Andy Shevchenko , Thomas Gleixner , Jann Horn , Feng Tang , Kees Cook , Randy Dunlap , Daniel Vetter Subject: Re: [PATCH 2/5] kernel.h: Add non_block_start/end() Message-ID: <20190815132127.GI9477@dhcp22.suse.cz> References: <20190814202027.18735-1-daniel.vetter@ffwll.ch> <20190814202027.18735-3-daniel.vetter@ffwll.ch> <20190814235805.GB11200@ziepe.ca> <20190815065829.GA7444@phenom.ffwll.local> <20190815122344.GA21596@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190815122344.GA21596@ziepe.ca> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 15-08-19 09:23:44, Jason Gunthorpe wrote: > On Thu, Aug 15, 2019 at 08:58:29AM +0200, Daniel Vetter wrote: > > On Wed, Aug 14, 2019 at 08:58:05PM -0300, Jason Gunthorpe wrote: > > > On Wed, Aug 14, 2019 at 10:20:24PM +0200, Daniel Vetter wrote: > > > > In some special cases we must not block, but there's not a > > > > spinlock, preempt-off, irqs-off or similar critical section already > > > > that arms the might_sleep() debug checks. Add a non_block_start/end() > > > > pair to annotate these. > > > > > > > > This will be used in the oom paths of mmu-notifiers, where blocking is > > > > not allowed to make sure there's forward progress. Quoting Michal: > > > > > > > > "The notifier is called from quite a restricted context - oom_reaper - > > > > which shouldn't depend on any locks or sleepable conditionals. The code > > > > should be swift as well but we mostly do care about it to make a forward > > > > progress. Checking for sleepable context is the best thing we could come > > > > up with that would describe these demands at least partially." > > > > > > But this describes fs_reclaim_acquire() - is there some reason we are > > > conflating fs_reclaim with non-sleeping? > > > > No idea why you tie this into fs_reclaim. We can definitly sleep in there, > > and for e.g. kswapd (which also wraps everything in fs_reclaim) we're > > event supposed to I thought. To make sure we can get at the last bit of > > memory by flushing all the queues and waiting for everything to be cleaned > > out. > > AFAIK the point of fs_reclaim is to prevent "indirect dependency upon > the page allocator" ie a justification that was given this !blockable > stuff. > > For instance: > > fs_reclaim_acquire() > kmalloc(GFP_KERNEL) <- lock dep assertion > > And further, Michal's concern about indirectness through locks is also > handled by lockdep: > > CPU0 CPU1 > mutex_lock() > kmalloc(GFP_KERNEL) > mutex_unlock() > fs_reclaim_acquire() > mutex_lock() <- lock dep assertion > > In other words, to prevent recursion into the page allocator you use > fs_reclaim_acquire(), and lockdep verfies it in its usual robust way. fs_reclaim_acquire is about FS/IO recursions IIUC. We are talking about any !GFP_NOWAIT allocation context here and any {in}direct dependency on it. Whether fs_reclaim_acquire can be reused for that I do not know because I am not familiar with the lockdep machinery enough > I asked Tejun about this once in regards to WQ_MEM_RECLAIM and he > explained that it means you can't call the allocator functions in a > way that would recurse into reclaim (ie instead use instead GFP_ATOMIC, or > tolerate allocation failure, or various other things). > > So, the reason I bring it up is half the justifications you posted for > blockable had to do with not recursing into reclaim and deadlocking, > and didn't seem to have much to do with blocking. > > I'm asking if *non-blocking* is really the requirement or if this is > just the usual 'do not deadlock on the allocator' thing reclaim paths > alread have? No, non-blocking is a very coarse approximation of what we really need. But it should give us even a stronger condition. Essentially any sleep other than a preemption shouldn't be allowed in that context. -- Michal Hocko SUSE Labs