Re: A couple of questions

[neil@tc-1-192.ariake.gol.ne.jp]

Hi Stephen,

Stephen C. Tweedie wrote:-
> Hi,
> 
[..snip..]
>
> > 2) The last 2 of the 3 branches to end_wp_page seem to me to be
> > impossible code paths.
> 
> > 	if (!pte_present(pte))
> > 		goto end_wp_page;
> > 	if (pte_write(pte))
> > 		goto end_wp_page;
> 
> No, the start of do_wp_page() looks like:
> 
> 	pte = *page_table;
> 	new_page = __get_free_page(GFP_USER);
> 
> and the get_free_page() call can block if we are out of memory, dropping
> the kernel lock in the process.  The page table can be modified by
> kswapd during this interval.

Thanks for your reply.  I think you've missed my point on this one.
The variable "pte" is set before calling __get_free_page(), and being
local cannot be modified by other processes.  Hence I still believe
the 2 branches shown are impossible, their negative having been the
condition for entering do_wp_page().

The case you mention is captured by the initial test

	if (pte_val(*page_table) != pte_val(pte))
		goto end_wp_page;

performed before the two above.  Do you agree?

Cheers,

Neil.
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