Re: [merged] proctxt-update-kernel-filesystem-proctxt-documentation.patch removed from -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 24 Jun 2009 08:45:03 +0200 Stefani Seibold <stefani@seibold.net> wrote:

> Am Dienstag, den 23.06.2009, 23:32 -0700 schrieb Andrew Morton:
> > On Wed, 24 Jun 2009 08:20:44 +0200 Stefani Seibold <stefani@seibold.net> wrote:
> > 
> > > what is with the associated
> > > procfs-provide-stack-information-for-threads-v08.patch
> > > patch?
> > > 
> > > There was no real objections against this patch, so why not merge it for
> > > 2.6.31?
> > 
> > Alexey pointed out that it doesn't actually work.
> 
> That is not true... it works. With my patch the kernel does exactly know
> where the thread stack is and therefor it is easy to determinate the
> associated map.
> 

On Tue, 16 Jun 2009 02:33:33 +0400 Alexey Dobriyan <adobriyan@gmail.com> wrote:

> On Mon, Jun 15, 2009 at 03:02:05PM -0700, akpm@linux-foundation.org wrote:
> >      procfs-provide-stack-information-for-threads-v08.patch
> > --- a/fs/proc/array.c~procfs-provide-stack-information-for-threads-v08
> 
> > +++ a/fs/proc/array.c
> > @@ -321,6 +321,54 @@ static inline void task_context_switch_c
> >  			p->nivcsw);
> >  }
> >  
> > +static inline unsigned long get_stack_usage_in_bytes(struct vm_area_struct *vma,
> > +					struct task_struct *p)
> > +{
> > +	unsigned long	i;
> > +	struct page	*page;
> > +	unsigned long	stkpage;
> > +
> > +	stkpage = KSTK_ESP(p) & PAGE_MASK;
> > +
> > +#ifdef CONFIG_STACK_GROWSUP
> > +	for (i = vma->vm_end; i-PAGE_SIZE > stkpage; i -= PAGE_SIZE) {
> > +
> > +		page = follow_page(vma, i-PAGE_SIZE, 0);
> 
> How can this work?
> 
> If stack page got swapped out, you'll get smaller than actual result.

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?