Re: changing mm->mmap_sem (was: Re: system call for process information?)

[torvalds@transmeta.com (Linus Torvalds)]

In article <Pine.LNX.4.21.0103181122480.13050-100000@imladris.rielhome.conectiva>,
Rik van Riel  <riel@conectiva.com.br> wrote:
>
>OK, I'll write some code to prevent multiple threads from
>stepping all over each other when they pagefault at the
>same address.
>
>What would be the preferred method of fixing this ?
>
>- fixing do_swap_page and all ->nopage functions

There is no need to fix gthe "nopage" functions. They never see the page
table directly anyway. 

So the only thing that _should_ be needed is to make sure that
do_no_page(), do_swap_page() and do_anonymous_page() will re-aquire the
mm->page_table_lock and undo their work if it turns out that the page
table entry is no longer empty.. 

(do_wp_page() should already be ok in this regard - it already does this
exactly because present pagetable entries can already race with kswapd. 
What we're adding is that _nonpresent_ page table entries can race with
multiple invocations of concurrent page faults)

>- hacking handle_mm_fault to make sure no overlapping
>  pagefaults will be served at the same time

No. The whole reason the rw_semaphores were done in the first place was
to allow page faults to happen concurrently to allow threaded
applictions to scale up even when faulting.

		Linus