Re: page refcounts?

[Christoph Lameter <clameter@engr.sgi.com>]

On Thu, 29 Dec 2005, Avishay Traeger wrote:

> How does the kernel determine when a page is unused?  I see that
> __get_free_pages() returns pages that have no refcount and no flags
> indicating that they should not be reclaimed.
> 
> The first page that is returned looks like:
> flags:0x80000000 mapping:00000000 mapcount:0 count:1
> 
> And the subsequent pages look like:
> flags:0x80000000 mapping:00000000 mapcount:0 count:0
> 
> I am printing this out exactly the way bad_page() does.  How does the
> kernel know not to mess around with pages that have no refcount or any
> flags to indicate that they are in use?  I've already searched google
> and checked a couple books, and couldn't find an answer.

The kernel knows that a page is free if count == 0 in the first page.
All pages of a higher order allocation are managed through the status of 
the first page.
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