linux-kernel - Re: [PATCH v6 1/2] mm: limit growth of 3% hardcoded other user reserve

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Date:	Thu, 28 Mar 2013 12:49:19 -0400
From:	Andrew Shewmaker <agshew@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	alan@...rguk.ukuu.org.uk, simon.jeons@...il.com,
	ric.masonn@...il.com
Subject: Re: [PATCH v6 1/2] mm: limit growth of 3% hardcoded other user
 reserve

On Wed, Mar 27, 2013 at 02:28:32PM -0700, Andrew Morton wrote:
> On Mon, 18 Mar 2013 17:44:42 -0400 Andrew Shewmaker <agshew@...il.com> wrote:
> 
> > Add user_reserve_kbytes knob.
> > 
> > Limit the growth of the memory reserved for other user
> > processes to min(3% current process size, user_reserve_pages).
> > 
> > user_reserve_pages defaults to min(3% free pages, 128MB)
> 
> That was an epic changelog ;)

I didn't want to err on the side of brevity again :)
But I definitely don't want to be annoying, so I'll work 
on being concise without sacrificing important detail.

> >
> > ...
> >
> > +int __meminit init_user_reserve(void)
> > +{
> > +	unsigned long free_kbytes;
> > +
> > +	free_kbytes = global_page_state(NR_FREE_PAGES) << (PAGE_SHIFT - 10);
> > +
> > +	sysctl_user_reserve_kbytes = min(free_kbytes / 32, 1UL << 17);
> > +	return 0;
> > +}
> > +module_init(init_user_reserve)
> 
> Problem is, the initial default values will become wrong if memory if
> hot-added or hot-removed.
> 
> That could be fixed up by appropriate use of
> register_memory_notifier(), but what would the notification handler do
> if the operator has modified the value?  Proportionally scale it?

If the operator changed it to a greater value, then I imagine that would
be because the applications they use to recover require a bigger reserve 
to function. Then proportionally scaling down the value on hot-removal 
might mean that the operator can't recover when they expected to be able 
to. Maybe the best thing would be to leave it be in that case, or 
print a message telling them they need to re-evaluate the reserve size?

I won't be able to look at this again until next week, but I'll work on a 
version that handles hot-addition and hot-removal.

Thanks,

Andrew
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