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Date: Wed, 24 Jun 2009 08:30:32 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Stefani Seibold <stefani@seibold.net>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
       Alexey Dobriyan <adobriyan@gmail.com>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>, Ingo Molnar <mingo@elte.hu>
Subject: Re: [merged]
 proctxt-update-kernel-filesystem-proctxt-documentation.patch removed from
 -mm tree
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On Wed, 24 Jun 2009 11:33:25 +0200 Stefani Seibold <stefani@seibold.net> wrote:

> > > Alexey's point is that follow_page() will return NULL if it hits a
> > > swapped-out stack page and the loop will exit, leading to an incorrect
> > > (ie: short) return value from get_stack_usage_in_bytes().
> > >
> > > Is this claim wrong?
> > 
> 
> No.
> 
> I digged in the kernel source and the only solution i found is to use
> the walk_page_range() like show_smap() in proc/fs/task_mmu.c.
> 
> Maybe there is an easier way, but i dont know.
> 
> So i would implement a similar function like smaps_pte_range() in
> proc/fs/task_mmu.c to detected the high water usage.

Perhaps we could enhance follow_page() so that it can tell the caller
when the target page is "virtually there", but swapped out.  Add a new
FOLL_SWAP, I guess.


How to communicate this back to the caller?  Perhaps add another
argument to follow_page(), perhaps return some magic value such as

#define FOLLOW_PAGE_SWAPPED_PAGE ((struct page *)1)

Adding the additional argument would be nicer.