Re: [Patch] mm/ksm.c is doing an unneeded _notify in write_protect_page.

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

On Thu, 11 Mar 2010, Robin Holt wrote:
> 
> ksm.c's write_protect_page implements a lockless means of verifying a
> page does not have any users of the page which are not accounted for via
> other kernel tracking means.  It does this by removing the writable pte
> with TLB flushes, checking the page_count against the total known users,
> and then using set_pte_at_notify to make it a read-only entry.
> 
> An unneeded mmu_notifier callout is made in the case where the known
> users does not match the page_count.  In that event, we are inserting
> the identical pte and there is no need for the set_pte_at_notify, but
> rather the simpler set_pte_at suffices.
> 
> To: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Robin Holt <holt@sgi.com>
> Acked-by: Izik Eidus <ieidus@redhat.com>
> Acked-by: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>

Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

> Cc: Chris Wright <chrisw@redhat.com>
> Cc: linux-mm@kvack.org
> 
> ---
> 
>  mm/ksm.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: ksm_remove_notify/mm/ksm.c
> ===================================================================
> --- ksm_remove_notify.orig/mm/ksm.c	2010-03-11 11:21:57.000000000 -0600
> +++ ksm_remove_notify/mm/ksm.c	2010-03-11 11:21:59.000000000 -0600
> @@ -751,7 +751,7 @@ static int write_protect_page(struct vm_
>  		 * page
>  		 */
>  		if (page_mapcount(page) + 1 + swapped != page_count(page)) {
> -			set_pte_at_notify(mm, addr, ptep, entry);
> +			set_pte_at(mm, addr, ptep, entry);
>  			goto out_unlock;
>  		}
>  		entry = pte_wrprotect(entry);

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