From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198])
	by kanga.kvack.org (Postfix) with ESMTP id E11826B025F
	for <linux-mm@kvack.org>; Tue, 29 Aug 2017 09:53:41 -0400 (EDT)
Received: by mail-wr0-f198.google.com with SMTP id w62so4827763wrc.9
        for <linux-mm@kvack.org>; Tue, 29 Aug 2017 06:53:41 -0700 (PDT)
Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15])
        by mx.google.com with ESMTPS id r201si1348078wme.36.2017.08.29.06.53.40
        for <linux-mm@kvack.org>
        (version=TLS1 cipher=AES128-SHA bits=128/128);
        Tue, 29 Aug 2017 06:53:40 -0700 (PDT)
Date: Tue, 29 Aug 2017 15:53:39 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH] mm, memory_hotplug: do not back off draining pcp free
 pages from kworker context
Message-ID: <20170829135339.svjhc6mt235rdlyd@dhcp22.suse.cz>
References: <20170828093341.26341-1-mhocko@kernel.org>
 <20170828153359.f9b252f99647eebd339a3a89@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170828153359.f9b252f99647eebd339a3a89@linux-foundation.org>
Sender: owner-linux-mm@kvack.org
List-ID: <linux-mm.kvack.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>, Tejun Heo <tj@kernel.org>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>

On Mon 28-08-17 15:33:59, Andrew Morton wrote:
> On Mon, 28 Aug 2017 11:33:41 +0200 Michal Hocko <mhocko@kernel.org> 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 <mhocko@suse.com>
> 
> No cc:stable?

I wouldn't be opposed I have just seen so many things broken in this
area I didn't consider this important enough. This would be 4.11+

Thanks!
-- 
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: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>