Re: [PATCH] mm, memory_hotplug: do not back off draining pcp free pages from kworker context

From: Andrew Morton <akpm@linux-foundation.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>, Tejun Heo <tj@kernel.org>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH] mm, memory_hotplug: do not back off draining pcp free pages from kworker context
Date: Mon, 28 Aug 2017 15:33:59 -0700	[thread overview]
Message-ID: <20170828153359.f9b252f99647eebd339a3a89@linux-foundation.org> (raw)
In-Reply-To: <20170828093341.26341-1-mhocko@kernel.org>

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>

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