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Date:	Mon, 5 Jan 2015 17:55:05 -0500
From:	Rafael Aquini <aquini@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, jweiner@...hat.com,
	dave.hansen@...ux.intel.com, rientjes@...gle.com,
	linux-mm@...ck.org
Subject: Re: [PATCH v2] fs: proc: task_mmu: show page size in
 /proc/<pid>/numa_maps

On Mon, Jan 05, 2015 at 01:35:00PM -0800, Andrew Morton wrote:
> On Mon,  5 Jan 2015 12:44:31 -0500 Rafael Aquini <aquini@...hat.com> wrote:
> 
> > This patch introduces 'kernelpagesize_kB' line element to /proc/<pid>/numa_maps
> > report file in order to help identifying the size of pages that are backing
> > memory areas mapped by a given task. This is specially useful to
> > help differentiating between HUGE and GIGANTIC page backed VMAs.
> > 
> > This patch is based on Dave Hansen's proposal and reviewer's follow-ups
> > taken from the following dicussion threads:
> >  * https://lkml.org/lkml/2011/9/21/454
> 
> Dave's changelog contains useful information which this one lacked.  I
> stole some of it.
> 
> : The output of /proc/$pid/numa_maps is in terms of number of pages like
> : anon=22 or dirty=54.  Here's some output:
> : 
> : 7f4680000000 default file=/hugetlb/bigfile anon=50 dirty=50 N0=50
> : 7f7659600000 default file=/anon_hugepage\040(deleted) anon=50 dirty=50 N0=50
> : 7fff8d425000 default stack anon=50 dirty=50 N0=50
> : Looks like we have a stack and a couple of anonymous hugetlbfs
> : areas page which both use the same amount of memory.  They don't.
> : 
> : The 'bigfile' uses 1GB pages and takes up ~50GB of space.  The
> : anon_hugepage uses 2MB pages and takes up ~100MB of space while the stack
> : uses normal 4k pages.  You can go over to smaps to figure out what the
> : page size _really_ is with KernelPageSize or MMUPageSize.  But, I think
> : this is a pretty nasty and counterintuitive interface as it stands.
> : 
> : This patch introduces 'kernelpagesize_kB' line element to
> : /proc/<pid>/numa_maps report file in order to help identifying the size of
> : pages that are backing memory areas mapped by a given task.  This is
> : specially useful to help differentiating between HUGE and GIGANTIC page
> : backed VMAs.
> : 
> : This patch is based on Dave Hansen's proposal and reviewer's follow-ups
> : taken from the following dicussion threads:
> :  * https://lkml.org/lkml/2011/9/21/454
> :  * https://lkml.org/lkml/2014/12/20/66
> 
> 
> > +	seq_printf(m, " kernelpagesize_kB=%lu", vma_kernel_pagesize(vma) >> 10);
> 
> This changes the format of the numa_maps file and can potentially break
> existing parsers.  Please discuss.
> 
> I'd complain about the patch's failure to update the documentation,
> except numa_maps appears to be undocumented.  Sigh.  What the heck is "N0"?
>
That's a nice opportunity to attempt to sharp my doc writing skills.
Sorry for the total failure to identify it earlier.
I just took it as a TODO note to send a patch to document this interface soon.

Happy new year.
-- Rafael
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