linux-kernel - Re: [PATCH] mm: add MM_SWAPENTS and page table when calculate tasksize in lowmem

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Date:	Wed, 17 Feb 2016 19:10:00 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Xishi Qiu <qiuxishi@...wei.com>, arve@...roid.com,
	riandrews@...roid.com, devel@...verdev.osuosl.org,
	zhong jiang <zhongjiang@...wei.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: add MM_SWAPENTS and page table when calculate
 tasksize in lowmem_scan()

On Tue 16-02-16 16:35:39, David Rientjes wrote:
> On Tue, 16 Feb 2016, Greg Kroah-Hartman wrote:
> 
> > On Tue, Feb 16, 2016 at 05:37:05PM +0800, Xishi Qiu wrote:
> > > Currently tasksize in lowmem_scan() only calculate rss, and not include swap.
> > > But usually smart phones enable zram, so swap space actually use ram.
> > 
> > Yes, but does that matter for this type of calculation?  I need an ack
> > from the android team before I could ever take such a core change to
> > this code...
> > 
> 
> The calculation proposed in this patch is the same as the generic oom 
> killer, it's an estimate of the amount of memory that will be freed if it 
> is killed and can exit.  This is better than simply get_mm_rss().
> 
> However, I think we seriously need to re-consider the implementation of 
> the lowmem killer entirely.  It currently abuses the use of TIF_MEMDIE, 
> which should ideally only be set for one thread on the system since it 
> allows unbounded access to global memory reserves.
> 
> It also abuses the user-visible /proc/self/oom_score_adj tunable: this 
> tunable is used by the generic oom killer to bias or discount a proportion 
> of memory from a process's usage.  This is the only supported semantic of 
> the tunable.  The lowmem killer uses it as a strict prioritization, so any 
> process with oom_score_adj higher than another process is preferred for 
> kill, REGARDLESS of memory usage.  This leads to priority inversion, the 
> user is unable to always define the same process to be killed by the 
> generic oom killer and the lowmem killer.  This is what happens when a 
> tunable with a very clear and defined purpose is used for other reasons.
> 
> I'd seriously consider not accepting any additional hacks on top of this 
> code until the implementation is rewritten.

Fully agreed!

-- 
Michal Hocko
SUSE Labs