question about COW

question about COW

[Sasha Sirotkin]

There is an "early COW" mechanism in __do_fault() which, if the page is 
not present and the fault is FAULT_PAGE_WRITE goes ahead and copies the 
page in order to prevent the next exception.

The question - why the code in __do_fault() does not decrease the shared 
map count of the old page as do_wp_page does ? And while we are at it, 
while this "early COW" code is much more simple than do_wp_page()?

Thanks.

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Re: question about COW

[KAMEZAWA Hiroyuki]

On Sat, 17 Apr 2010 10:07:26 +0300
Sasha Sirotkin <buildroot@browserseal.com> wrote:

> There is an "early COW" mechanism in __do_fault() which, if the page is 
> not present and the fault is FAULT_PAGE_WRITE goes ahead and copies the 
> page in order to prevent the next exception.
> 
> The question - why the code in __do_fault() does not decrease the shared 
> map count of the old page as do_wp_page does ? And while we are at it, 
> while this "early COW" code is much more simple than do_wp_page()?
> 
> Thanks.
> 
IIUC.

Case 1) A task cause a write page fault because pte is not set as PRESENT.
        __do_fault() is called. And the kernel found vma is not-SHARED.

       Do eary-COW. In this case, the old page was not _mapped_...IOW, the task's
       this pte was not accounted into old_page->mapcount.
       We just increase new_page->mapcount. Don't touch old_page->mapcount.

Case 2) A task caused a write page fault because pte was not WRITABLE.
       
       do_wp_page() is called because the page was _mapped_.
       If the page is shared, decrease old_page->mapcount, increae new_page->mapcount.

Regard,
-Kame

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