linux-kernel - Re: [PATCH 1/2] Report the pagesize backing a VMA in /proc/pid/smaps

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Date:	Mon, 22 Sep 2008 17:21:52 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] Report the pagesize backing a VMA in /proc/pid/smaps

On (22/09/08 08:55), Dave Hansen didst pronounce:
> On Mon, 2008-09-22 at 02:38 +0100, Mel Gorman wrote:
> > It is useful to verify that a hugepage-aware application is using the expected
> > pagesizes in each of its memory regions. This patch reports the pagesize
> > backing the VMA in /proc/pid/smaps. This should not break any sensible
> > parser as the file format is multi-line and it should skip information it
> > does not recognise.
> 
> Time to play devil's advocate. :)
> 
> To be fair, this doesn't return the MMU pagesize backing the VMA.  It
> returns pagesize that hugetlb reports *or* the kernel's base PAGE_SIZE.
> 

True. In the vast majority of cases, this is the MMU size with ppc64 on
pro

> The ppc64 case where we have a 64k PAGE_SIZE, but no hardware 64k
> support means that we'll have a 4k MMU pagesize that we're pretending is
> a 64k MMU page.  That might confuse someone seeing 16x the number of TLB
> misses they expect.

The corollary is that someone running with a 64K base page kernel may be
surprised that the pagesize is always 4K. However I'll check if there is
a simple way of checking out if the MMU size differs from PAGE_SIZE.

> This also doesn't work if, in the future, we get multiple page sizes
> mapped under one VMA.  But, I guess that all only matters if you worry
> about how the kernel is treating the pages vs. the MMU hardware.
> 

Will deal with that problem if and when we encounter it. It may be a
case that VMAs split or that we could report how many pages of each MMU
size are in that VMA.

Thanks


-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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