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

From: Carsten Otte
Date: Mon Jun 20 2005 - 06:23:02 EST

On 6/20/05, Mauricio Lin <mauriciolin@xxxxxxxxx> wrote:
> 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
> page.
> 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?
Franky, one cannot predict which pages will get copied on
write since that is related to the behaviour of the process:
if the process writes, then the corresponding page gets copied.
If the vma has the VM_WRITE flag set, and the pte is read-only,
the page is a candidate to become subject to copy on write.
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