linux-kernel - Re: [RFC 1/3] /dev/low_mem

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Date:	Wed, 18 Jan 2012 09:16:49 +0200 (EET)
From:	Pekka Enberg <penberg@...nel.org>
To:	Minchan Kim <minchan@...nel.org>
cc:	Rik van Riel <riel@...hat.com>, linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, leonid.moiseichuk@...ia.com,
	kamezawa.hiroyu@...fujitsu.com, mel@....ul.ie, rientjes@...gle.com,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ronen Hod <rhod@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC 1/3] /dev/low_mem_notify

On Wed, 18 Jan 2012, Minchan Kim wrote:
> I didn't look into your code(will do) but as I read description,
> still I don't convince we need really some process specific threshold like 99%
> I think application can know it by polling /proc/meminfo without this mechanism
> if they really want.

I'm not sure if we need arbitrary threshold either. However, we need to 
support the following cases:

   - We're about to swap

   - We're about to run out of memory

   - We're about to start OOM killing

and I don't think your patch solves that. One possibility is to implement:

   VMNOTIFY_TYPE_ABOUT_TO_SWAP
   VMNOTIFY_TYPE_ABOUT_TO_OOM
   VMNOTIFY_TYPE_ABOUT_TO_OOM_KILL

and maybe rip out support for arbitrary thresholds. Does that more 
reasonable?

As for polling /proc/meminfo, I'd much rather deliver stats as part of 
vmnotify_read() because it's easier to extend the ABI rather than adding 
new fields to /proc/meminfo.

On Wed, 18 Jan 2012, Minchan Kim wrote:
> I would like to notify when system has a trobule with memory pressure without
> some process specific threshold. Of course, applicatoin can't expect it.(ie,
> application can know system memory pressure by /proc/meminfo but it can't know
> when swapout really happens). Kernel low mem notify have to give such notification
> to user space, I think.

It should be simple to add support for VMNOTIFY_TYPE_MEM_PRESSURE that 
uses your hooks.

 			Pekka
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