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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D304DC433F5 for ; Thu, 25 Nov 2021 08:48:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349031AbhKYIvx (ORCPT ); Thu, 25 Nov 2021 03:51:53 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:40620 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1352490AbhKYItv (ORCPT ); Thu, 25 Nov 2021 03:49:51 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 1B12821B36; Thu, 25 Nov 2021 08:46:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1637830000; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=cswIxoSlRlT63La6eOMo8YzRcdw09aN70/ZppeCVAcc=; b=nbG0J+pLA5AfBgyD3YVer5rSFRseQoG/ODIGX9H6d34Q2d/QfevBhdszMSYRPztsqvFq8o 4arZ3AgMy7iJrwNY93edMm1iAE3FjJG/GI9nlpjT8ywie2QZZsoTg5jcx0sT499stoIEWP ay9fUr14Aoue2DyQdUP7QZ3/KYgJZdU= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id E3892A3B81; Thu, 25 Nov 2021 08:46:39 +0000 (UTC) Date: Thu, 25 Nov 2021 09:46:39 +0100 From: Michal Hocko To: Uladzislau Rezki Cc: Andrew Morton , Dave Chinner , Neil Brown , Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, LKML , Ilya Dryomov , Jeff Layton Subject: Re: [PATCH v2 2/4] mm/vmalloc: add support for __GFP_NOFAIL Message-ID: References: <20211122153233.9924-1-mhocko@kernel.org> <20211122153233.9924-3-mhocko@kernel.org> <20211123170238.f0f780ddb800f1316397f97c@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 24-11-21 21:11:42, Uladzislau Rezki wrote: > On Tue, Nov 23, 2021 at 05:02:38PM -0800, Andrew Morton wrote: > > On Tue, 23 Nov 2021 20:01:50 +0100 Uladzislau Rezki wrote: > > > > > On Mon, Nov 22, 2021 at 04:32:31PM +0100, Michal Hocko wrote: > > > > From: Michal Hocko > > > > > > > > Dave Chinner has mentioned that some of the xfs code would benefit from > > > > kvmalloc support for __GFP_NOFAIL because they have allocations that > > > > cannot fail and they do not fit into a single page. > > > > Perhaps we should tell xfs "no, do it internally". Because this is a > > rather nasty-looking thing - do we want to encourage other callsites to > > start using it? > > > > > > The large part of the vmalloc implementation already complies with the > > > > given gfp flags so there is no work for those to be done. The area > > > > and page table allocations are an exception to that. Implement a retry > > > > loop for those. > > > > > > > > Add a short sleep before retrying. 1 jiffy is a completely random > > > > timeout. Ideally the retry would wait for an explicit event - e.g. > > > > a change to the vmalloc space change if the failure was caused by > > > > the space fragmentation or depletion. But there are multiple different > > > > reasons to retry and this could become much more complex. Keep the retry > > > > simple for now and just sleep to prevent from hogging CPUs. > > > > > > > > Yes, the horse has already bolted. But we didn't want that horse anyway ;) > > > > I added GFP_NOFAIL back in the mesozoic era because quite a lot of > > sites were doing open-coded try-forever loops. I thought "hey, they > > shouldn't be doing that in the first place, but let's at least > > centralize the concept to reduce code size, code duplication and so > > it's something we can now grep for". But longer term, all GFP_NOFAIL > > sites should be reworked to no longer need to do the retry-forever > > thing. In retrospect, this bright idea of mine seems to have added > > license for more sites to use retry-forever. Sigh. > > > > > > + if (nofail) { > > > > + schedule_timeout_uninterruptible(1); > > > > + goto again; > > > > + } > > > > The idea behind congestion_wait() is to prevent us from having to > > hard-wire delays like this. congestion_wait(1) would sleep for up to > > one millisecond, but will return earlier if reclaim events happened > > which make it likely that the caller can now proceed with the > > allocation event, successfully. > > > > However it turns out that congestion_wait() was quietly broken at the > > block level some time ago. We could perhaps resurrect the concept at > > another level - say by releasing congestion_wait() callers if an amount > > of memory newly becomes allocatable. This obviously asks for inclusion > > of zone/node/etc info from the congestion_wait() caller. But that's > > just an optimization - if the newly-available memory isn't useful to > > the congestion_wait() caller, they just fail the allocation attempts > > and wait again. > > > > > well that is sad... > > > I have raised two concerns in our previous discussion about this change, > > > > Can you please reiterate those concerns here? > > > 1. I proposed to repeat(if fails) in one solid place, i.e. get rid of > duplication and spreading the logic across several places. This is about > simplification. I am all for simplifications. But the presented simplification lead to 2) and ... > 2. Second one is about to do an unwinding and release everything what we > have just accumulated in terms of memory consumption. The failure might > occur, if so a condition we are in is a low memory one or high memory > pressure. In this case, since we are about to sleep some milliseconds > in order to repeat later, IMHO it makes sense to release memory: > > - to prevent killing apps or possible OOM; > - we can end up looping quite a lot of time or even forever if users do > nasty things with vmalloc API and __GFP_NOFAIL flag. ... this is where we disagree and I have tried to explain why. The primary memory to allocate are pages to back the vmalloc area. Failing to allocate few page tables - which btw. do not fail as they are order-0 - and result into the whole and much more expensive work to allocate the former is really wasteful. You've had a concern about OOM killer invocation while retrying the page table allocation but you should realize that page table allocations might already invoke OOM killer so that is absolutely nothing new. -- Michal Hocko SUSE Labs