How to identify cow (copy-on-write) pages during kernel execution?

From: Mauricio Lin
Date: Mon Jun 20 2005 - 00:46:46 EST

Hi all,

I would like to know if there is a way to identify
struct page that is cow (copy-on-write).

The way I figured out to identify cow pages is when
copy-on-write happens. I mean I identify cow pages
inside the do_wp_page(), the function that handles
copy-on-write. I have checked do_no_page() as well.

I have included a field (is_cow) in the struct page to
identify cow page.

struct page {
atomic_t is_cow;

But I wonder if it is possible to identify cow pages
before copy-on-write happens. So identify cow pages in
advance before any process tries to write to a cow

I have checked the do_fork(), copy_process() and
copy_mm() function to try to identify cow pages during
the process creation, but no success. In copy_mm(),
just the mm (of current process) is provided to the
child process, but there are no references to struct
pages related to mm and its VMAs.

So when a page struct is considered a cow in the
kernel and its count variable is updated? Certainly
the counter page (page->_count) is updated when a page
is shared because of copy-on-write feature.
How can I identify cow pages when it becomes cow? Is
there any feasible way to perform that?


Mauricio Lin.
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