linux-kernel - Re: [PATCH 1/3] procfs: Export next

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Date:	Mon, 30 Jan 2012 23:51:06 +0400
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	San Mehat <san@...gle.com>, Colin Cross <ccross@...roid.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	kernel-team@...roid.com, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 1/3] procfs: Export next_tgid(), move it to kernel/pid.c

On Sun, Jan 29, 2012 at 07:26:55PM -0800, Eric W. Biederman wrote:
> Anton Vorontsov <anton.vorontsov@...aro.org> writes:
> 
> > We'd like to use this function in the android low memory killer driver, so
> > let's export it.
> >
> > Also, move next_tgid() to kernel/pid.c, so now it lives with the rest of
> > pid library functions and does not depend on procfs. Plus, we may now hide
> > find_ge_pid() from the global namespace.
> >
> > While at it, also turn next_tgid()'s comments into kerneldoc format.
> >
> > There should be no functional changes.
> 
> Ouch no.
> 
> There is find_ge_pid that is general purpose, and there are a lot of
> things with the task list.  But this iterator is specific to the
> implementation of proc especially in how it grabs and drops locks.

Exactly, the way procfs grabs and drops locks is very attractive for
us to reuse in lowmemorykiller driver. :-) Proc is efficient in a
way that it does not need to hold the tacklist lock (otherwise, I guess,
it could be prone to attacks, e.g. by constantly readdiring /proc/ ?).

> Using this code outside of /proc will make /proc harder to maintain than
> it already is.
> 
> Why in the world would you want to traverse processes by pids when
> there are well defined iterators for looping over tasks already.

We just did not want to hold the global tasklist lock, if we could
avoid it. Low memory killer is a low priority stuff, and grabbing
tasklist lock seemed inefficient.

Thanks,

-- 
Anton Vorontsov
Email: cbouatmailru@...il.com
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