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

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Message-Id: <1222284190.15523.64.camel@nimitz>
Date:	Wed, 24 Sep 2008 12:23:10 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>, agl@...ibm.com,
	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 Wed, 2008-09-24 at 20:11 +0100, Mel Gorman wrote:
> I don't get what you mean by it being sprinkled in each smaps file. How
> would you present the data?

1. figure out what the file path is from smaps
2. look up the mount
3. look up the page sizes from the mount's information

> > We should be able to figure out which
> > mount the file is from and, from there, maybe we need some per-mount
> > information exported.  
> 
> Per-mount information is already exported and you can infer the data about
> huge pagesizes. For example, if you know the default huge pagesize (from
> /proc/meminfo), and the file is on hugetlbfs (read maps, then /proc/mounts)
> and there is no pagesize= mount option (mounts again), you could guess what the
> hugepage that is backing a VMA is. Shared memory segments are a little harder
> but again, you can infer the information if you look around for long enough.
> 
> However, this is awkward and not very user-friendly. With the patches (minus
> MMUPageSize as I think we've agreed to postpone that), it's easy to see what
> pagesize is being used at a glance. Without it, you need to know a fair bit
> about hugepages are implemented in Linux to infer the information correctly.

I agree completely.  But, if we consider this a user ABI thing, then
we're stuck with it for a long time, and we better make it flexible
enough to at least contain the gunk we're planning on adding in a small
number of years, like the fallback.  We don't want to be adding this
stuff if it isn't going to be stable.

-- Dave

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