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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 D54E6C433E0 for ; Thu, 18 Mar 2021 08:10:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8C74064F45 for ; Thu, 18 Mar 2021 08:10:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229649AbhCRIJh (ORCPT ); Thu, 18 Mar 2021 04:09:37 -0400 Received: from mx2.suse.de ([195.135.220.15]:34716 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229540AbhCRIJG (ORCPT ); Thu, 18 Mar 2021 04:09:06 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1616054945; 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=AYbYHzxzMIJD7TqPSeyrbmTDs2X+aT9qDOtxlKAHE2s=; b=DB7oESWT+Ryk/R9i0wJiVKnWYyAMJUTLOcSfnf0uHB4ImAz675V2H3hBp5P4iXqGU/s1T1 p9MaUvgMfRV3rJWDtuvcUGh9nuFdA1/K8YavJlfudfMjZ2jXa7Vv1h7B/NHhNqj8wNL5Ys XfmnrDphhUkMYPmnIw9FvxkUq+h24nY= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 1ADB1AC17; Thu, 18 Mar 2021 08:09:05 +0000 (UTC) Date: Thu, 18 Mar 2021 09:09:01 +0100 From: Michal Hocko To: Andrew Morton Cc: Minchan Kim , linux-mm , LKML , joaodias@google.com, surenb@google.com, cgoldswo@codeaurora.org, willy@infradead.org, david@redhat.com, vbabka@suse.cz, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v3 2/3] mm: disable LRU pagevec during the migration temporarily Message-ID: References: <20210310161429.399432-1-minchan@kernel.org> <20210310161429.399432-2-minchan@kernel.org> <20210317171316.d261de806203d8d99c6bf0ef@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210317171316.d261de806203d8d99c6bf0ef@linux-foundation.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 17-03-21 17:13:16, Andrew Morton wrote: > On Wed, 10 Mar 2021 08:14:28 -0800 Minchan Kim wrote: > > > LRU pagevec holds refcount of pages until the pagevec are drained. > > It could prevent migration since the refcount of the page is greater > > than the expection in migration logic. To mitigate the issue, > > callers of migrate_pages drains LRU pagevec via migrate_prep or > > lru_add_drain_all before migrate_pages call. > > > > However, it's not enough because pages coming into pagevec after the > > draining call still could stay at the pagevec so it could keep > > preventing page migration. Since some callers of migrate_pages have > > retrial logic with LRU draining, the page would migrate at next trail > > but it is still fragile in that it doesn't close the fundamental race > > between upcoming LRU pages into pagvec and migration so the migration > > failure could cause contiguous memory allocation failure in the end. > > > > To close the race, this patch disables lru caches(i.e, pagevec) > > during ongoing migration until migrate is done. > > > > Since it's really hard to reproduce, I measured how many times > > migrate_pages retried with force mode(it is about a fallback to a > > sync migration) with below debug code. > > > > int migrate_pages(struct list_head *from, new_page_t get_new_page, > > .. > > .. > > > > if (rc && reason == MR_CONTIG_RANGE && pass > 2) { > > printk(KERN_ERR, "pfn 0x%lx reason %d\n", page_to_pfn(page), rc); > > dump_page(page, "fail to migrate"); > > } > > > > The test was repeating android apps launching with cma allocation > > in background every five seconds. Total cma allocation count was > > about 500 during the testing. With this patch, the dump_page count > > was reduced from 400 to 30. > > > > The new interface is also useful for memory hotplug which currently > > drains lru pcp caches after each migration failure. This is rather > > suboptimal as it has to disrupt others running during the operation. > > With the new interface the operation happens only once. This is also in > > line with pcp allocator cache which are disabled for the offlining as > > well. > > > > This is really a rather ugly thing, particularly from a maintainability > point of view. Are you sure you found all the sites which need the > enable/disable? How do we prevent new ones from creeping in which need > the same treatment? Is there some way of adding a runtime check which > will trip if a conversion was missed? I am not sure I am following. What is your concern here? This is a lock-like interface to disable a certain optimization because it stands in the way. Not using the interface is not a correctness problem. If you refer to disable/enable interface and one potentially missing enable for some reason then again this will not become a correctness problem. It will result in a suboptimal behavior. So in the end this is much less of a probel than leaving a lock behind. The functionality is not exported to modules and I would agree that this is not something for out of core/MM code to be used. We can hide it into an internal mm header of you want? -- Michal Hocko SUSE Labs