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Date: Fri, 1 Mar 2013 19:08:28 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
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To: Simon Jeons <simon.jeons@gmail.com>
cc: Johannes Weiner <hannes@cmpxchg.org>, dormando <dormando@rydia.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Seiji Aguchi <seiji.aguchi@hds.com>,
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        "lwoodman@redhat.com" <lwoodman@redhat.com>,
        Mel Gorman <mel@csn.ul.ie>
Subject: Re: [PATCH] add extra free kbytes tunable
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On Sat, 2 Mar 2013, Simon Jeons wrote:
> On 03/02/2013 09:42 AM, Hugh Dickins wrote:
> > On Sat, 2 Mar 2013, Simon Jeons wrote:
> > > In function __add_to_swap_cache if add to radix tree successfully will
> > > result
> > > in increase NR_FILE_PAGES, why? This is anonymous page instead of file
> > > backed
> > > page.
> > Right, that's hard to understand without historical background.
> > 
> > I think the quick answer would be that we used to (and still do) think
> > of file-cache and swap-cache as two halves of page-cache.  And then when
> 
> shmem page should be treated as file-cache or swap-cache? It is strange since
> it is consist of anonymous pages and these pages establish files.

A shmem page is swap-backed file-cache, and it may get transferred to or
from swap-cache: yes, it's a difficult and confusing case, as I said below.

I would never call it "anonymous", but it is counted in /proc/meminfo's
Active(anon) or Inactive(anon) rather than in (file), because "anon"
there is shorthand for "swap-backed".

> > So you'll find that shmem and swap are counted as file in some places
> > and anon in others, and it's hard to grasp which is where and why,
> > without remembering the history.

Hugh