linux-kernel - Re: What does the PG_swapbacked of page flags actually mean?

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Date:	Thu, 21 Feb 2013 10:25:20 +0800
From:	common An <xx.kernel@...il.com>
To:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linaro-kernel@...ts.linaro.org, riel@...hat.com
Subject: Re: What does the PG_swapbacked of page flags actually mean?

On Wed, Feb 20, 2013 at 6:43 PM, common An <xx.kernel@...il.com> wrote:
> PG_swapbacked is a bit for page->flags.
>
> In kernel code, its comment is "page is backed by RAM/swap". But I couldn't
> understand it.
> 1. Does the RAM mean DRAM? How page is backed by RAM?
> 2. When the page is page-out to swap file, the bit PG_swapbacked will be set
> to demonstrate this page is backed by swap. Is it right?
> 3. In general, when will call SetPageSwapBacked() to set the bit?

>From : http://www.gossamer-threads.com/lists/linux/kernel/840692#840692

Every anonymous, tmpfs or shared memory segment page is potentially
swap backed. That is the whole point of the PG_swapbacked flag.

A page from a filesystem like ext3 or NFS cannot suddenly turn into
a swap backed page. This page "nature" is not changed during the
lifetime of a page.

But, I am still a little confusing.

>
> Could anybody kindly explain for me?
>
> Thanks very much.
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