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From: Mel Gorman on 28 May 2010 06:10
On Thu, May 27, 2010 at 01:43:00PM -0700, Doug Doan wrote:
> From: Doug Doan <dougd(a)cray.com>
>
> When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
> through handle_pte_fault for normal pages, or through hugetlb_fault for
> huge pages.
>
> In the normal page case, we eventually get to do_wp_page and call mmu
> notifiers via ptep_clear_flush_notify. There is no callout to the mmmu
> notifiers in the huge page case. This patch fixes that.
>
> Signed-off-by: Doug Doan <dougd(a)cray.com>
> ---

> --- mm/hugetlb.c.orig 2010-05-27 13:07:58.569546314 -0700
> +++ mm/hugetlb.c 2010-05-26 14:41:06.449296524 -0700
> @@ -2345,11 +2345,17 @@ retry_avoidcopy:
> ptep = huge_pte_offset(mm, address & huge_page_mask(h));
> if (likely(pte_same(huge_ptep_get(ptep), pte))) {
> /* Break COW */
> + mmu_notifier_invalidate_range_start(mm,
> + address & huge_page_mask(h),
> + (address & huge_page_mask(h)) + huge_page_size(h));

Should the address not already be aligned?

Otherwise, I don't see any problem.

> huge_ptep_clear_flush(vma, address, ptep);
> set_huge_pte_at(mm, address, ptep,
> make_huge_pte(vma, new_page, 1));
> /* Make the old page be freed below */
> new_page = old_page;
> + mmu_notifier_invalidate_range_end(mm,
> + address & huge_page_mask(h),
> + (address & huge_page_mask(h)) + huge_page_size(h));
> }
> page_cache_release(new_page);
> page_cache_release(old_page);


--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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From: Doug Doan on 28 May 2010 12:50
On 05/28/2010 02:59 AM, Mel Gorman wrote:
> On Thu, May 27, 2010 at 01:43:00PM -0700, Doug Doan wrote:
>> From: Doug Doan<dougd(a)cray.com>
>>
>> When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
>> through handle_pte_fault for normal pages, or through hugetlb_fault for
>> huge pages.
>>
>> In the normal page case, we eventually get to do_wp_page and call mmu
>> notifiers via ptep_clear_flush_notify. There is no callout to the mmmu
>> notifiers in the huge page case. This patch fixes that.
>>
>> Signed-off-by: Doug Doan<dougd(a)cray.com>
>> ---
>
>> --- mm/hugetlb.c.orig 2010-05-27 13:07:58.569546314 -0700
>> +++ mm/hugetlb.c 2010-05-26 14:41:06.449296524 -0700
>> @@ -2345,11 +2345,17 @@ retry_avoidcopy:
>> ptep = huge_pte_offset(mm, address& huge_page_mask(h));
>> if (likely(pte_same(huge_ptep_get(ptep), pte))) {
>> /* Break COW */
>> + mmu_notifier_invalidate_range_start(mm,
>> + address& huge_page_mask(h),
>> + (address& huge_page_mask(h)) + huge_page_size(h));
>
> Should the address not already be aligned?

I'm not seeing where the address was aligned before this point. The code just
above aligns it:

ptep = huge_pte_offset(mm, address& huge_page_mask(h));
if (likely(pte_same(huge_ptep_get(ptep), pte))) {
/* Break COW */

> Otherwise, I don't see any problem.
>
>> huge_ptep_clear_flush(vma, address, ptep);
>> set_huge_pte_at(mm, address, ptep,
>> make_huge_pte(vma, new_page, 1));
>> /* Make the old page be freed below */
>> new_page = old_page;
>> + mmu_notifier_invalidate_range_end(mm,
>> + address& huge_page_mask(h),
>> + (address& huge_page_mask(h)) + huge_page_size(h));
>> }
>> page_cache_release(new_page);
>> page_cache_release(old_page);
>
>
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From: Mel Gorman on 28 May 2010 13:40
On Fri, May 28, 2010 at 09:39:59AM -0700, Doug Doan wrote:
> On 05/28/2010 02:59 AM, Mel Gorman wrote:
>> On Thu, May 27, 2010 at 01:43:00PM -0700, Doug Doan wrote:
>>> From: Doug Doan<dougd(a)cray.com>
>>>
>>> When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
>>> through handle_pte_fault for normal pages, or through hugetlb_fault for
>>> huge pages.
>>>
>>> In the normal page case, we eventually get to do_wp_page and call mmu
>>> notifiers via ptep_clear_flush_notify. There is no callout to the mmmu
>>> notifiers in the huge page case. This patch fixes that.
>>>
>>> Signed-off-by: Doug Doan<dougd(a)cray.com>
>>> ---
>>
>>> --- mm/hugetlb.c.orig 2010-05-27 13:07:58.569546314 -0700
>>> +++ mm/hugetlb.c 2010-05-26 14:41:06.449296524 -0700
>>> @@ -2345,11 +2345,17 @@ retry_avoidcopy:
>>> ptep = huge_pte_offset(mm, address& huge_page_mask(h));
>>> if (likely(pte_same(huge_ptep_get(ptep), pte))) {
>>> /* Break COW */
>>> + mmu_notifier_invalidate_range_start(mm,
>>> + address& huge_page_mask(h),
>>> + (address& huge_page_mask(h)) + huge_page_size(h));
>>
>> Should the address not already be aligned?
>
> I'm not seeing where the address was aligned before this point. The code
> just above aligns it:
>
> ptep = huge_pte_offset(mm, address& huge_page_mask(h));
> if (likely(pte_same(huge_ptep_get(ptep), pte))) {
> /* Break COW */
>

I'm sorry. You're right. I was looking at the copy_huge_page which was
not aligning the address. It should be but it's ultimately harmless as
the parameter is discarded.

Acked-by: Mel Gorman <mel(a)csn.ul.ie>

>> Otherwise, I don't see any problem.
>>
>>> huge_ptep_clear_flush(vma, address, ptep);
>>> set_huge_pte_at(mm, address, ptep,
>>> make_huge_pte(vma, new_page, 1));
>>> /* Make the old page be freed below */
>>> new_page = old_page;
>>> + mmu_notifier_invalidate_range_end(mm,
>>> + address& huge_page_mask(h),
>>> + (address& huge_page_mask(h)) + huge_page_size(h));
>>> }
>>> page_cache_release(new_page);
>>> page_cache_release(old_page);
>>
>>
>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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From: Andrew Morton on 2 Jun 2010 02:20
On Thu, 27 May 2010 13:43:00 -0700 Doug Doan <dougd(a)cray.com> wrote:

>
> When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
> through handle_pte_fault for normal pages, or through hugetlb_fault for huge pages.
>
> In the normal page case, we eventually get to do_wp_page and call mmu notifiers
> via ptep_clear_flush_notify. There is no callout to the mmmu notifiers in the
> huge page case. This patch fixes that.
>
> Signed-off-by: Doug Doan <dougd(a)cray.com>
> ---
>
> [patch text/plain (802B)]
> --- mm/hugetlb.c.orig 2010-05-27 13:07:58.569546314 -0700
> +++ mm/hugetlb.c 2010-05-26 14:41:06.449296524 -0700

(In patch -p1 form, please. So a/mm/hugetlb.c)

> @@ -2345,11 +2345,17 @@ retry_avoidcopy:
> ptep = huge_pte_offset(mm, address & huge_page_mask(h));
> if (likely(pte_same(huge_ptep_get(ptep), pte))) {
> /* Break COW */
> + mmu_notifier_invalidate_range_start(mm,
> + address & huge_page_mask(h),
> + (address & huge_page_mask(h)) + huge_page_size(h));
> huge_ptep_clear_flush(vma, address, ptep);
> set_huge_pte_at(mm, address, ptep,
> make_huge_pte(vma, new_page, 1));
> /* Make the old page be freed below */
> new_page = old_page;
> + mmu_notifier_invalidate_range_end(mm,
> + address & huge_page_mask(h),
> + (address & huge_page_mask(h)) + huge_page_size(h));
> }
> page_cache_release(new_page);
> page_cache_release(old_page);

This causes mmu_notifier_invalidate_range_start() to be called under
page_table_lock. The immediately preceding code seems to take some
care to avoid doing that. I took a quick look at other callsites and
cannot immediately see other cases where
mmu_notifier_invalidate_range_start/end() are called under that lock.

This may not introduce bugs with current notifier implementations (I
didn't check), but it does lessen flexibility?

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From: Andrew Morton on 2 Jun 2010 19:40
On Wed, 2 Jun 2010 16:13:42 -0700
Doug Doan <dougd(a)cray.com> wrote:

> On 06/01/2010 11:16 PM, Andrew Morton wrote:
> > On Thu, 27 May 2010 13:43:00 -0700 Doug Doan<dougd(a)cray.com> wrote:
> >
> >>
> >> When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
> >> through handle_pte_fault for normal pages, or through hugetlb_fault for huge pages.
> >>
> >> In the normal page case, we eventually get to do_wp_page and call mmu notifiers
> >> via ptep_clear_flush_notify. There is no callout to the mmmu notifiers in the
> >> huge page case. This patch fixes that.
> >>
> >> Signed-off-by: Doug Doan<dougd(a)cray.com>
> >> ---
> >>
> >> [patch text/plain (802B)]
> >> --- mm/hugetlb.c.orig 2010-05-27 13:07:58.569546314 -0700
> >> +++ mm/hugetlb.c 2010-05-26 14:41:06.449296524 -0700
> >
> > (In patch -p1 form, please. So a/mm/hugetlb.c)
> >
> >> @@ -2345,11 +2345,17 @@ retry_avoidcopy:
> >> ptep = huge_pte_offset(mm, address& huge_page_mask(h));
> >> if (likely(pte_same(huge_ptep_get(ptep), pte))) {
> >> /* Break COW */
> >> + mmu_notifier_invalidate_range_start(mm,
> >> + address& huge_page_mask(h),
> >> + (address& huge_page_mask(h)) + huge_page_size(h));
> >> huge_ptep_clear_flush(vma, address, ptep);
> >> set_huge_pte_at(mm, address, ptep,
> >> make_huge_pte(vma, new_page, 1));
> >> /* Make the old page be freed below */
> >> new_page = old_page;
> >> + mmu_notifier_invalidate_range_end(mm,
> >> + address& huge_page_mask(h),
> >> + (address& huge_page_mask(h)) + huge_page_size(h));
> >> }
> >> page_cache_release(new_page);
> >> page_cache_release(old_page);
> >
> > This causes mmu_notifier_invalidate_range_start() to be called under
> > page_table_lock. The immediately preceding code seems to take some
> > care to avoid doing that. I took a quick look at other callsites and
> > cannot immediately see other cases where
> > mmu_notifier_invalidate_range_start/end() are called under that lock.
> >
> > This may not introduce bugs with current notifier implementations (I
> > didn't check), but it does lessen flexibility?
>
> In the normal page case, handle_pte_fault calls do_wp_page inside a spinlock on
> ptl = pte_lockptr(mm, pmd), which uses mm->page_table_lock if USE_SPLIT_PTLOCKS
> is not defined.
>
> I don't understand what you mean by lessen flexibilty.

Well, specifically it means that
mmu_notifier_invalidate_range_start/end() implemetnations can no longer
take page_table_lock or any lock which nests outside page_table_lock.
That lessens flexibility.

As the other mmu_notifier_invalidate_range_start/end() callsite in this
function carefully nested those calls outside page_table_lock, perhaps
that was thought to be a significant thing.
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