Re: [RFC 1/3] /dev/low_mem_notify

[Pekka Enberg]

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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