What does the PG_swapbacked of page flags actually mean?

What does the PG_swapbacked of page flags actually mean?

[common An]

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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?

Could anybody kindly explain for me?

Thanks very much.

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What does the PG_swapbacked of page flags actually mean?

[common An]

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?

Could anybody kindly explain for me?

Thanks very much.

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

[common An]

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