linux-kernel - Re: quicklists confuse meminfo

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Message-ID: <20080309111456.GA21690@elte.hu>
Date:	Sun, 9 Mar 2008 12:14:56 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	Bart Van Assche <bart.vanassche@...il.com>
Subject: Re: quicklists confuse meminfo


* Thomas Gleixner <tglx@...utronix.de> wrote:

> Bart reported http://bugzilla.kernel.org/show_bug.cgi?id=9991. He 
> assumed a memory leak in 32bit kernels when he analyzed the output of 
> /proc/meminfo.
> 
> The leak is not a leak, it's an accounting bug. quicklists keep a 
> large amount of pages which are accounted as used memory.
[...]
> Another strange observation about quicklists is the imbalance of the 
> quicklists across CPUs. Running the above loop on a 2way machine I can 
> observe that the quicklist pages are acuumulating on one CPU. Stopping 
> and restarting the loop a couple of times can shift the accumulation 
> from one to the other CPU.

hm. I think we should not let this much RAM hang around in a 
special-purpose allocator like quicklists. Shouldnt the quicklists be 
temporary in nature, and be trimmed much more agressively?

in fact, we have a check_pgt_cache() call in cpu_idle(), which does:

        quicklist_trim(0, pgd_dtor, 25, 16);

but it appears we dont do quicklist trimming anywhere else! So if a 
system has no idle time, the quicklist can grow unbounded, and that's a 
real memory leak IMO.

	Ingo
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