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

From: common An
Date: Wed Feb 20 2013 - 21:25:35 EST

On Wed, Feb 20, 2013 at 6:43 PM, common An <xx.kernel@xxxxxxxxx> 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 :

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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