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

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

[Robin Holt]

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.

Signed-off-by: Robin Holt <holt@sgi.com>
To: Izik Eidus <ieidus@redhat.com>
Cc: 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 09:24:30.000000000 -0600
+++ ksm_remove_notify/mm/ksm.c	2010-03-11 09:35:18.000000000 -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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Re: [Patch] mm/ksm.c is doing an unneeded _notify in write_protect_page.

[Izik Eidus]

On Thu, 11 Mar 2010 09:54:22 -0600
Robin Holt <holt@sgi.com> 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.
> 
> Signed-off-by: Robin Holt <holt@sgi.com>
> To: Izik Eidus <ieidus@redhat.com>
> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> Cc: Chris Wright <chrisw@redhat.com>
> Cc: linux-mm@kvack.org

Acked-by: Izik Eidus <ieidus@redhat.com>

> 
> ---
> 
>  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 09:24:30.000000000 -0600
> +++ ksm_remove_notify/mm/ksm.c	2010-03-11 09:35:18.000000000 -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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Re: [Patch] mm/ksm.c is doing an unneeded _notify in write_protect_page.

[Andrea Arcangeli]

On Thu, Mar 11, 2010 at 06:01:59PM +0200, Izik Eidus wrote:
> On Thu, 11 Mar 2010 09:54:22 -0600
> Robin Holt <holt@sgi.com> 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.
> > 
> > Signed-off-by: Robin Holt <holt@sgi.com>
> > To: Izik Eidus <ieidus@redhat.com>
> > Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> > Cc: Chris Wright <chrisw@redhat.com>
> > Cc: linux-mm@kvack.org
> 
> Acked-by: Izik Eidus <ieidus@redhat.com>

Ack too. I misunderstood what you were talking about before, patch
makes it clear now ;).

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

[Robin Holt]

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

[Hugh Dickins]

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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