[PATCH] mm/hugetlb: Use the right pte val for compare in hugetlb_cow

[PATCH] mm/hugetlb: Use the right pte val for compare in hugetlb_cow

[Aneesh Kumar K.V]

We cannot use the pte value used in set_pte_at for pte_same comparison,
because archs like ppc64, filter/add new pte flag in set_pte_at. Instead
fetch the pte value inside hugetlb_cow. We are comparing pte value to
make sure the pte didn't change since we dropped the page table lock.
hugetlb_cow get called with page table lock held, and we can take a copy
of the pte value before we drop the page table lock.

With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no
previous mapping (huge_pte_none entries), by forcing a cow in the fault
path. This avoid take an addition fault to covert a read-only mapping
to read/write. Here we were comparing a recently instantiated pte (via
set_pte_at) to the pte values from linux page table. As explained above
on ppc64 such pte_same check returned wrong result, resulting in us
taking an additional fault on ppc64.

Fixes: 6a119eae942c ("powerpc/mm: Add a _PAGE_PTE bit")

Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
---
 mm/hugetlb.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ec49d9ef1eef..da8fbd02b92e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3386,15 +3386,17 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
  * Keep the pte_same checks anyway to make transition from the mutex easier.
  */
 static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
-			unsigned long address, pte_t *ptep, pte_t pte,
-			struct page *pagecache_page, spinlock_t *ptl)
+		       unsigned long address, pte_t *ptep,
+		       struct page *pagecache_page, spinlock_t *ptl)
 {
+	pte_t pte;
 	struct hstate *h = hstate_vma(vma);
 	struct page *old_page, *new_page;
 	int ret = 0, outside_reserve = 0;
 	unsigned long mmun_start;	/* For mmu_notifiers */
 	unsigned long mmun_end;		/* For mmu_notifiers */
 
+	pte = huge_ptep_get(ptep);
 	old_page = pte_page(pte);
 
 retry_avoidcopy:
@@ -3668,7 +3670,7 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	hugetlb_count_add(pages_per_huge_page(h), mm);
 	if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
 		/* Optimization, do the COW without a second fault */
-		ret = hugetlb_cow(mm, vma, address, ptep, new_pte, page, ptl);
+		ret = hugetlb_cow(mm, vma, address, ptep, page, ptl);
 	}
 
 	spin_unlock(ptl);
@@ -3822,8 +3824,8 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	if (flags & FAULT_FLAG_WRITE) {
 		if (!huge_pte_write(entry)) {
-			ret = hugetlb_cow(mm, vma, address, ptep, entry,
-					pagecache_page, ptl);
+			ret = hugetlb_cow(mm, vma, address, ptep,
+					  pagecache_page, ptl);
 			goto out_put_page;
 		}
 		entry = huge_pte_mkdirty(entry);
-- 
2.10.1

Re: [PATCH] mm/hugetlb: Use the right pte val for compare in hugetlb_cow

[Andrew Morton]

On Tue, 18 Oct 2016 21:12:45 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

> We cannot use the pte value used in set_pte_at for pte_same comparison,
> because archs like ppc64, filter/add new pte flag in set_pte_at. Instead
> fetch the pte value inside hugetlb_cow. We are comparing pte value to
> make sure the pte didn't change since we dropped the page table lock.
> hugetlb_cow get called with page table lock held, and we can take a copy
> of the pte value before we drop the page table lock.
> 
> With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no
> previous mapping (huge_pte_none entries), by forcing a cow in the fault
> path. This avoid take an addition fault to covert a read-only mapping
> to read/write. Here we were comparing a recently instantiated pte (via
> set_pte_at) to the pte values from linux page table. As explained above
> on ppc64 such pte_same check returned wrong result, resulting in us
> taking an additional fault on ppc64.

>From my reading this is a minor performance improvement and a -stable
backport isn't needed.  But it is unclear whether the impact warrants a
4.9 merge.

Please be careful about describing end-user visible impacts when fixing
bugs, thanks.

Re: [PATCH] mm/hugetlb: Use the right pte val for compare in hugetlb_cow

[Hillf Danton]

On Tuesday, October 18, 2016 11:43 PM Aneesh Kumar K.V wrote:
> 
> We cannot use the pte value used in set_pte_at for pte_same comparison,
> because archs like ppc64, filter/add new pte flag in set_pte_at. Instead
> fetch the pte value inside hugetlb_cow. We are comparing pte value to
> make sure the pte didn't change since we dropped the page table lock.
> hugetlb_cow get called with page table lock held, and we can take a copy
> of the pte value before we drop the page table lock.
> 
> With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no
> previous mapping (huge_pte_none entries), by forcing a cow in the fault
> path. This avoid take an addition fault to covert a read-only mapping
> to read/write. Here we were comparing a recently instantiated pte (via
> set_pte_at) to the pte values from linux page table. As explained above
> on ppc64 such pte_same check returned wrong result, resulting in us
> taking an additional fault on ppc64.
> 
> Fixes: 6a119eae942c ("powerpc/mm: Add a _PAGE_PTE bit")
> 
> Reported-by: Jan Stancek <jstancek@redhat.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---

Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>

>  mm/hugetlb.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ec49d9ef1eef..da8fbd02b92e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3386,15 +3386,17 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
>   * Keep the pte_same checks anyway to make transition from the mutex easier.
>   */
>  static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
> -			unsigned long address, pte_t *ptep, pte_t pte,
> -			struct page *pagecache_page, spinlock_t *ptl)
> +		       unsigned long address, pte_t *ptep,
> +		       struct page *pagecache_page, spinlock_t *ptl)
>  {
> +	pte_t pte;
>  	struct hstate *h = hstate_vma(vma);
>  	struct page *old_page, *new_page;
>  	int ret = 0, outside_reserve = 0;
>  	unsigned long mmun_start;	/* For mmu_notifiers */
>  	unsigned long mmun_end;		/* For mmu_notifiers */
> 
> +	pte = huge_ptep_get(ptep);
>  	old_page = pte_page(pte);
> 
>  retry_avoidcopy:
> @@ -3668,7 +3670,7 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	hugetlb_count_add(pages_per_huge_page(h), mm);
>  	if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
>  		/* Optimization, do the COW without a second fault */
> -		ret = hugetlb_cow(mm, vma, address, ptep, new_pte, page, ptl);
> +		ret = hugetlb_cow(mm, vma, address, ptep, page, ptl);
>  	}
> 
>  	spin_unlock(ptl);
> @@ -3822,8 +3824,8 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> 
>  	if (flags & FAULT_FLAG_WRITE) {
>  		if (!huge_pte_write(entry)) {
> -			ret = hugetlb_cow(mm, vma, address, ptep, entry,
> -					pagecache_page, ptl);
> +			ret = hugetlb_cow(mm, vma, address, ptep,
> +					  pagecache_page, ptl);
>  			goto out_put_page;
>  		}
>  		entry = huge_pte_mkdirty(entry);
> --
> 2.10.1

Re: [PATCH] mm/hugetlb: Use the right pte val for compare in hugetlb_cow

[Aneesh Kumar K.V]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Tue, 18 Oct 2016 21:12:45 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
>
>> We cannot use the pte value used in set_pte_at for pte_same comparison,
>> because archs like ppc64, filter/add new pte flag in set_pte_at. Instead
>> fetch the pte value inside hugetlb_cow. We are comparing pte value to
>> make sure the pte didn't change since we dropped the page table lock.
>> hugetlb_cow get called with page table lock held, and we can take a copy
>> of the pte value before we drop the page table lock.
>> 
>> With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no
>> previous mapping (huge_pte_none entries), by forcing a cow in the fault
>> path. This avoid take an addition fault to covert a read-only mapping
>> to read/write. Here we were comparing a recently instantiated pte (via
>> set_pte_at) to the pte values from linux page table. As explained above
>> on ppc64 such pte_same check returned wrong result, resulting in us
>> taking an additional fault on ppc64.
>
> From my reading this is a minor performance improvement and a -stable
> backport isn't needed.  But it is unclear whether the impact warrants a
> 4.9 merge.

This patch workaround the issue reported at https://lkml.kernel.org/r/57FF7BB4.1070202@redhat.com
The reason for that OOM was a reserve count accounting issue which
happens in the error path of hugetlb_cow. Not this patch avoid us taking
the error path and hence we don't have the reported OOM.

An actual fix for that issue is being worked on by Mike Kravetz.

>
> Please be careful about describing end-user visible impacts when fixing
> bugs, thanks.

-aneesh