linux-kernel - Re: [PATCH v2 11/51] memcg: implement mem_cgroup_css_from

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Message-ID: <20150527170955.GA25324@cmpxchg.org>
Date:	Wed, 27 May 2015 13:09:55 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org, jack@...e.cz,
	hch@...radead.org, linux-fsdevel@...r.kernel.org,
	vgoyal@...hat.com, lizefan@...wei.com, cgroups@...r.kernel.org,
	linux-mm@...ck.org, mhocko@...e.cz, clm@...com,
	fengguang.wu@...el.com, david@...morbit.com, gthelen@...gle.com,
	khlebnikov@...dex-team.ru
Subject: Re: [PATCH v2 11/51] memcg: implement mem_cgroup_css_from_page()

On Wed, May 27, 2015 at 12:13:44PM -0400, Tejun Heo wrote:
> From 26bab580abfc441c841c1983469b8b86f5a8ef5c Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@...nel.org>
> Date: Wed, 27 May 2015 12:08:29 -0400
> 
> Implement mem_cgroup_css_from_page() which returns the
> cgroup_subsys_state of the memcg associated with a given page.  This
> will be used by cgroup writeback support.
> 
> This function assumes that page->mem_cgroup association doesn't change
> until the page is released, which is true on the default hierarchy as
> long as mem_cgroup_migrate() is not used.  As the only user of
> mem_cgroup_migrate() is FUSE which won't support cgroup writeback for
> the time being, this works for now, and mem_cgroup_migrate() will soon
> be updated so that the invariant actually holds.

Regular page migration uses mem_cgroup_migrate() as well, but it's not
a problem as it ensures that the old page doesn't have any outstanding
references at that point.

It's only replace_page_cache_page() that calls mem_cgroup_migrate() on
a live page breaking mem_cgroup_css_from_page().

So the page looks fine, I'd just update the culprit function in the
changelog and kerneldoc.
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