From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f197.google.com (mail-wr0-f197.google.com [209.85.128.197]) by kanga.kvack.org (Postfix) with ESMTP id AF4A86B02C3 for ; Tue, 29 Aug 2017 07:28:27 -0400 (EDT) Received: by mail-wr0-f197.google.com with SMTP id p37so4372132wrc.5 for ; Tue, 29 Aug 2017 04:28:27 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id o20si2134103wrg.477.2017.08.29.04.28.26 for (version=TLS1 cipher=AES128-SHA bits=128/128); Tue, 29 Aug 2017 04:28:26 -0700 (PDT) Date: Tue, 29 Aug 2017 13:28:23 +0200 From: Michal Hocko Subject: Re: [PATCH] mm, memory_hotplug: do not back off draining pcp free pages from kworker context Message-ID: <20170829112823.GA12413@dhcp22.suse.cz> References: <20170828093341.26341-1-mhocko@kernel.org> <20170828153359.f9b252f99647eebd339a3a89@linux-foundation.org> <6e138348-aa28-8660-d902-96efafe1dcb2@I-love.SAKURA.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6e138348-aa28-8660-d902-96efafe1dcb2@I-love.SAKURA.ne.jp> Sender: owner-linux-mm@kvack.org List-ID: To: Tetsuo Handa , Tejun Heo Cc: Andrew Morton , Mel Gorman , linux-mm@kvack.org, LKML On Tue 29-08-17 20:20:39, Tetsuo Handa wrote: > On 2017/08/29 7:33, Andrew Morton wrote: > > On Mon, 28 Aug 2017 11:33:41 +0200 Michal Hocko wrote: > > > >> drain_all_pages backs off when called from a kworker context since > >> 0ccce3b924212 ("mm, page_alloc: drain per-cpu pages from workqueue > >> context") because the original IPI based pcp draining has been replaced > >> by a WQ based one and the check wanted to prevent from recursion and > >> inter workers dependencies. This has made some sense at the time > >> because the system WQ has been used and one worker holding the lock > >> could be blocked while waiting for new workers to emerge which can be a > >> problem under OOM conditions. > >> > >> Since then ce612879ddc7 ("mm: move pcp and lru-pcp draining into single > >> wq") has moved draining to a dedicated (mm_percpu_wq) WQ with a rescuer > >> so we shouldn't depend on any other WQ activity to make a forward > >> progress so calling drain_all_pages from a worker context is safe as > >> long as this doesn't happen from mm_percpu_wq itself which is not the > >> case because all workers are required to _not_ depend on any MM locks. > >> > >> Why is this a problem in the first place? ACPI driven memory hot-remove > >> (acpi_device_hotplug) is executed from the worker context. We end > >> up calling __offline_pages to free all the pages and that requires > >> both lru_add_drain_all_cpuslocked and drain_all_pages to do their job > >> otherwise we can have dangling pages on pcp lists and fail the offline > >> operation (__test_page_isolated_in_pageblock would see a page with 0 > >> ref. count but without PageBuddy set). > >> > >> Fix the issue by removing the worker check in drain_all_pages. > >> lru_add_drain_all_cpuslocked doesn't have this restriction so it works > >> as expected. > >> > >> Fixes: 0ccce3b924212 ("mm, page_alloc: drain per-cpu pages from workqueue context") > >> Signed-off-by: Michal Hocko > > > > No cc:stable? > > > > Michal, are you sure that this patch does not cause deadlock? > > As shown in "[PATCH] mm: Use WQ_HIGHPRI for mm_percpu_wq." thread, currently work > items on mm_percpu_wq seem to be blocked by other work items not on mm_percpu_wq. But we have a rescuer so we should make a forward progress eventually. Or am I missing something. Tejun, could you have a look please?/ -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org