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.