linux-kernel - Re: [PATCH 1/3] mm: migrate: do not touch page->mem

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Message-ID: <20160203092009.GE21016@esperanza>
Date:	Wed, 3 Feb 2016 12:20:09 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<kernel-team@...com>
Subject: Re: [PATCH 1/3] mm: migrate: do not touch page->mem_cgroup of live
 pages

On Fri, Jan 29, 2016 at 06:19:31PM -0500, Johannes Weiner wrote:
> Changing a page's memcg association complicates dealing with the page,
> so we want to limit this as much as possible. Page migration e.g. does
> not have to do that. Just like page cache replacement, it can forcibly
> charge a replacement page, and then uncharge the old page when it gets
> freed. Temporarily overcharging the cgroup by a single page is not an
> issue in practice, and charging is so cheap nowadays that this is much
> preferrable to the headache of messing with live pages.
> 
> The only place that still changes the page->mem_cgroup binding of live
> pages is when pages move along with a task to another cgroup. But that
> path isolates the page from the LRU, takes the page lock, and the move
> lock (lock_page_memcg()). That means page->mem_cgroup is always stable
> in callers that have the page isolated from the LRU or locked. Lighter
> unlocked paths, like writeback accounting, can use lock_page_memcg().
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Vladimir Davydov <vdavydov@...tuozzo.com>