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

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Date:	Tue, 16 Feb 2016 16:35:39 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
cc:	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 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.