[PATCH 0/1] Clarify huge_pte_offset() semantics

[PATCH 0/1] Clarify huge_pte_offset() semantics

[Punit Agrawal]

Hi,

The following patch is an attempt to make huge_pte_offset() consistent
when dealing with different levels of the page table and document the
expected semantics. Previously posting can be found at [0].

Changelog

RFC - v1
* Merge Patch 1 and 2 - preserve bisectability
* Drop RFC tag

Original cover letter follows...

The generic implementation of huge_pte_offset() has inconsistent
behaviour when looking up hugepage PUDs vs PMDs entries that are not
present (returning NULL vs pte_t*).

Similarly, it returns NULL when encountering swap entries although all
the callers have special checks to properly deal with swap entries.

Without clear semantics, it is difficult to determine if a change
breaks huge_pte_offset() without going through all the scenarios where
it is used.

I faced this recently when updating the arm64 implementation of
huge_pte_offset() to handle swap entries (related to enabling poisoned
memeory)[1]. And will come across again when I update it for
contiguous hugepage support now that core changes have been merged.

To address these issues, this following patch -

* makes huge_pte_offset() consistent between PUD and PMDs
* and, documents the expected behaviour of huge_pte_offset()

All feedback welcome.

Thanks,
Punit

[0] https://lkml.org/lkml/2017/7/24/514
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f02ab08afbe76ee7b0b2a34a9970e7dd200d8b01

Punit Agrawal (1):
  mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

 mm/hugetlb.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

-- 
2.11.0

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[PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Punit Agrawal]

When walking the page tables to resolve an address that points to
!p*d_present() entry, huge_pte_offset() returns inconsistent values
depending on the level of page table (PUD or PMD).

It returns NULL in the case of a PUD entry while in the case of a PMD
entry, it returns a pointer to the page table entry.

A similar inconsitency exists when handling swap entries - returns NULL
for a PUD entry while a pointer to the pte_t is retured for the PMD
entry.

Update huge_pte_offset() to make the behaviour consistent - return NULL
in the case of p*d_none() and a pointer to the pte_t for hugepage or
swap entries.

Document the behaviour to clarify the expected behaviour of this
function. This is to set clear semantics for architecture specific
implementations of huge_pte_offset().

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
---
 mm/hugetlb.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bc48ee783dd9..72dd1139a8e4 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4603,6 +4603,13 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
 	return pte;
 }
 
+/*
+ * huge_pte_offset() - Walk the page table to resolve the hugepage
+ * entry at address @addr
+ *
+ * Return: Pointer to page table or swap entry (PUD or PMD) for address @addr
+ * or NULL if the entry is p*d_none().
+ */
 pte_t *huge_pte_offset(struct mm_struct *mm,
 		       unsigned long addr, unsigned long sz)
 {
@@ -4617,13 +4624,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
 	p4d = p4d_offset(pgd, addr);
 	if (!p4d_present(*p4d))
 		return NULL;
+
 	pud = pud_offset(p4d, addr);
-	if (!pud_present(*pud))
+	if (pud_none(*pud))
 		return NULL;
-	if (pud_huge(*pud))
+	/* hugepage or swap? */
+	if (pud_huge(*pud) || !pud_present(*pud))
 		return (pte_t *)pud;
+
 	pmd = pmd_offset(pud, addr);
-	return (pte_t *) pmd;
+	if (pmd_none(*pmd))
+		return NULL;
+	/* hugepage or swap? */
+	if (pmd_huge(*pmd) || !pmd_present(*pmd))
+		return (pte_t *) pmd;
+
+	return NULL;
 }
 
 #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
-- 
2.11.0

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Catalin Marinas]

On Tue, Jul 25, 2017 at 04:41:14PM +0100, Punit Agrawal wrote:
> When walking the page tables to resolve an address that points to
> !p*d_present() entry, huge_pte_offset() returns inconsistent values
> depending on the level of page table (PUD or PMD).
> 
> It returns NULL in the case of a PUD entry while in the case of a PMD
> entry, it returns a pointer to the page table entry.
> 
> A similar inconsitency exists when handling swap entries - returns NULL
> for a PUD entry while a pointer to the pte_t is retured for the PMD
> entry.
> 
> Update huge_pte_offset() to make the behaviour consistent - return NULL
> in the case of p*d_none() and a pointer to the pte_t for hugepage or
> swap entries.
> 
> Document the behaviour to clarify the expected behaviour of this
> function. This is to set clear semantics for architecture specific
> implementations of huge_pte_offset().
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Michal Hocko]

On Tue 25-07-17 16:41:14, Punit Agrawal wrote:
> When walking the page tables to resolve an address that points to
> !p*d_present() entry, huge_pte_offset() returns inconsistent values
> depending on the level of page table (PUD or PMD).
> 
> It returns NULL in the case of a PUD entry while in the case of a PMD
> entry, it returns a pointer to the page table entry.
> 
> A similar inconsitency exists when handling swap entries - returns NULL
> for a PUD entry while a pointer to the pte_t is retured for the PMD
> entry.
> 
> Update huge_pte_offset() to make the behaviour consistent - return NULL
> in the case of p*d_none() and a pointer to the pte_t for hugepage or
> swap entries.
> 
> Document the behaviour to clarify the expected behaviour of this
> function. This is to set clear semantics for architecture specific
> implementations of huge_pte_offset().

hugetlb pte semantic is a disaster and I agree it could see some
cleanup/clarifications but I am quite nervous to see a patchi like this.
How do we check that nothing will get silently broken by this change?

> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> ---
>  mm/hugetlb.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bc48ee783dd9..72dd1139a8e4 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4603,6 +4603,13 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>  	return pte;
>  }
>  
> +/*
> + * huge_pte_offset() - Walk the page table to resolve the hugepage
> + * entry at address @addr
> + *
> + * Return: Pointer to page table or swap entry (PUD or PMD) for address @addr
> + * or NULL if the entry is p*d_none().
> + */
>  pte_t *huge_pte_offset(struct mm_struct *mm,
>  		       unsigned long addr, unsigned long sz)
>  {
> @@ -4617,13 +4624,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>  	p4d = p4d_offset(pgd, addr);
>  	if (!p4d_present(*p4d))
>  		return NULL;
> +
>  	pud = pud_offset(p4d, addr);
> -	if (!pud_present(*pud))
> +	if (pud_none(*pud))
>  		return NULL;
> -	if (pud_huge(*pud))
> +	/* hugepage or swap? */
> +	if (pud_huge(*pud) || !pud_present(*pud))
>  		return (pte_t *)pud;
> +
>  	pmd = pmd_offset(pud, addr);
> -	return (pte_t *) pmd;
> +	if (pmd_none(*pmd))
> +		return NULL;
> +	/* hugepage or swap? */
> +	if (pmd_huge(*pmd) || !pmd_present(*pmd))
> +		return (pte_t *) pmd;
> +
> +	return NULL;
>  }
>  
>  #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
> -- 
> 2.11.0
> 

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Michal Hocko]

On Wed 26-07-17 10:50:38, Michal Hocko wrote:
> On Tue 25-07-17 16:41:14, Punit Agrawal wrote:
> > When walking the page tables to resolve an address that points to
> > !p*d_present() entry, huge_pte_offset() returns inconsistent values
> > depending on the level of page table (PUD or PMD).
> > 
> > It returns NULL in the case of a PUD entry while in the case of a PMD
> > entry, it returns a pointer to the page table entry.
> > 
> > A similar inconsitency exists when handling swap entries - returns NULL
> > for a PUD entry while a pointer to the pte_t is retured for the PMD
> > entry.
> > 
> > Update huge_pte_offset() to make the behaviour consistent - return NULL
> > in the case of p*d_none() and a pointer to the pte_t for hugepage or
> > swap entries.
> > 
> > Document the behaviour to clarify the expected behaviour of this
> > function. This is to set clear semantics for architecture specific
> > implementations of huge_pte_offset().
> 
> hugetlb pte semantic is a disaster and I agree it could see some
> cleanup/clarifications but I am quite nervous to see a patchi like this.
> How do we check that nothing will get silently broken by this change?

Forgot to add. Hugetlb have been special because of the pte sharing. I
haven't looked into that code for quite some time but there might be a
good reason why pud behave differently.
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Punit Agrawal]

Hi Michal,

Michal Hocko <mhocko@kernel.org> writes:

> On Wed 26-07-17 10:50:38, Michal Hocko wrote:
>> On Tue 25-07-17 16:41:14, Punit Agrawal wrote:
>> > When walking the page tables to resolve an address that points to
>> > !p*d_present() entry, huge_pte_offset() returns inconsistent values
>> > depending on the level of page table (PUD or PMD).
>> > 
>> > It returns NULL in the case of a PUD entry while in the case of a PMD
>> > entry, it returns a pointer to the page table entry.
>> > 
>> > A similar inconsitency exists when handling swap entries - returns NULL
>> > for a PUD entry while a pointer to the pte_t is retured for the PMD
>> > entry.
>> > 
>> > Update huge_pte_offset() to make the behaviour consistent - return NULL
>> > in the case of p*d_none() and a pointer to the pte_t for hugepage or
>> > swap entries.
>> > 
>> > Document the behaviour to clarify the expected behaviour of this
>> > function. This is to set clear semantics for architecture specific
>> > implementations of huge_pte_offset().
>> 
>> hugetlb pte semantic is a disaster and I agree it could see some
>> cleanup/clarifications but I am quite nervous to see a patchi like this.
>> How do we check that nothing will get silently broken by this change?

Glad I'm not the only one who finds the hugetlb semantics somewhat
confusing. :)

I've been running tests from mce-test suite and libhugetlbfs for similar
changes we did on arm64. There could be assumptions that were not
exercised but I'm not sure how to check for all the possible usages.

Do you have any other suggestions that can help improve confidence in
the patch?

>
> Forgot to add. Hugetlb have been special because of the pte sharing. I
> haven't looked into that code for quite some time but there might be a
> good reason why pud behave differently.

I checked the code and don't see anything that would explain (or
require) the difference in behaviour.

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Michal Hocko]

On Wed 26-07-17 13:11:46, Punit Agrawal wrote:
> Hi Michal,
> 
> Michal Hocko <mhocko@kernel.org> writes:
> 
> > On Wed 26-07-17 10:50:38, Michal Hocko wrote:
> >> On Tue 25-07-17 16:41:14, Punit Agrawal wrote:
> >> > When walking the page tables to resolve an address that points to
> >> > !p*d_present() entry, huge_pte_offset() returns inconsistent values
> >> > depending on the level of page table (PUD or PMD).
> >> > 
> >> > It returns NULL in the case of a PUD entry while in the case of a PMD
> >> > entry, it returns a pointer to the page table entry.
> >> > 
> >> > A similar inconsitency exists when handling swap entries - returns NULL
> >> > for a PUD entry while a pointer to the pte_t is retured for the PMD
> >> > entry.
> >> > 
> >> > Update huge_pte_offset() to make the behaviour consistent - return NULL
> >> > in the case of p*d_none() and a pointer to the pte_t for hugepage or
> >> > swap entries.
> >> > 
> >> > Document the behaviour to clarify the expected behaviour of this
> >> > function. This is to set clear semantics for architecture specific
> >> > implementations of huge_pte_offset().
> >> 
> >> hugetlb pte semantic is a disaster and I agree it could see some
> >> cleanup/clarifications but I am quite nervous to see a patchi like this.
> >> How do we check that nothing will get silently broken by this change?
> 
> Glad I'm not the only one who finds the hugetlb semantics somewhat
> confusing. :)

This is a huge understatement. It is a source of nightmares.

> I've been running tests from mce-test suite and libhugetlbfs for similar
> changes we did on arm64. There could be assumptions that were not
> exercised but I'm not sure how to check for all the possible usages.
> 
> Do you have any other suggestions that can help improve confidence in
> the patch?

Unfortunatelly I don't. I just know there were many subtle assumptions
all over the place so I am rather careful to not touch the code unless
really necessary.

That being said, I am not opposing your patch.

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Michal Hocko]

On Wed 26-07-17 14:33:57, Michal Hocko wrote:
> On Wed 26-07-17 13:11:46, Punit Agrawal wrote:
[...]
> > I've been running tests from mce-test suite and libhugetlbfs for similar
> > changes we did on arm64. There could be assumptions that were not
> > exercised but I'm not sure how to check for all the possible usages.
> > 
> > Do you have any other suggestions that can help improve confidence in
> > the patch?
> 
> Unfortunatelly I don't. I just know there were many subtle assumptions
> all over the place so I am rather careful to not touch the code unless
> really necessary.
> 
> That being said, I am not opposing your patch.

Let me be more specific. I am not opposing your patch but we should
definitely need more reviewers to have a look. I am not seeing any
immediate problems with it but I do not see a large improvements either
(slightly less nightmare doesn't make me sleep all that well ;)). So I
will leave the decisions to others.
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Punit Agrawal]

Michal Hocko <mhocko@kernel.org> writes:

> On Wed 26-07-17 14:33:57, Michal Hocko wrote:
>> On Wed 26-07-17 13:11:46, Punit Agrawal wrote:
> [...]
>> > I've been running tests from mce-test suite and libhugetlbfs for similar
>> > changes we did on arm64. There could be assumptions that were not
>> > exercised but I'm not sure how to check for all the possible usages.
>> > 
>> > Do you have any other suggestions that can help improve confidence in
>> > the patch?
>> 
>> Unfortunatelly I don't. I just know there were many subtle assumptions
>> all over the place so I am rather careful to not touch the code unless
>> really necessary.
>> 
>> That being said, I am not opposing your patch.
>
> Let me be more specific. I am not opposing your patch but we should
> definitely need more reviewers to have a look. I am not seeing any
> immediate problems with it but I do not see a large improvements either
> (slightly less nightmare doesn't make me sleep all that well ;)). So I
> will leave the decisions to others.

I hear you - I'd definitely appreciate more eyes on the code change and
description.

Thanks for taking a look.

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Mike Kravetz]

On 07/26/2017 06:34 AM, Punit Agrawal wrote:
> Michal Hocko <mhocko@kernel.org> writes:
> 
>> On Wed 26-07-17 14:33:57, Michal Hocko wrote:
>>> On Wed 26-07-17 13:11:46, Punit Agrawal wrote:
>> [...]
>>>> I've been running tests from mce-test suite and libhugetlbfs for similar
>>>> changes we did on arm64. There could be assumptions that were not
>>>> exercised but I'm not sure how to check for all the possible usages.
>>>>
>>>> Do you have any other suggestions that can help improve confidence in
>>>> the patch?
>>>
>>> Unfortunatelly I don't. I just know there were many subtle assumptions
>>> all over the place so I am rather careful to not touch the code unless
>>> really necessary.
>>>
>>> That being said, I am not opposing your patch.
>>
>> Let me be more specific. I am not opposing your patch but we should
>> definitely need more reviewers to have a look. I am not seeing any
>> immediate problems with it but I do not see a large improvements either
>> (slightly less nightmare doesn't make me sleep all that well ;)). So I
>> will leave the decisions to others.
> 
> I hear you - I'd definitely appreciate more eyes on the code change and
> description.

I like the change in semantics for the routine.  Like you, I examined all
callers of huge_pte_offset() and it appears that they will not be impacted
by your change.

My only concern is that arch specific versions of huge_pte_offset, may
not (yet) follow the new semantic.  Someone could potentially introduce
a new huge_pte_offset call and depend on the new 'documented' semantics.
Yet, an unmodified arch specific version of huge_pte_offset might have
different semantics.  I have not reviewed all the arch specific instances
of the routine to know if this is even possible.  Just curious if you
examined these, or perhaps you think this is not an issue?

-- 
Mike Kravetz

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Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Punit Agrawal]

Mike Kravetz <mike.kravetz@oracle.com> writes:

> On 07/26/2017 06:34 AM, Punit Agrawal wrote:
>> Michal Hocko <mhocko@kernel.org> writes:
>> 
>>> On Wed 26-07-17 14:33:57, Michal Hocko wrote:
>>>> On Wed 26-07-17 13:11:46, Punit Agrawal wrote:
>>> [...]
>>>>> I've been running tests from mce-test suite and libhugetlbfs for similar
>>>>> changes we did on arm64. There could be assumptions that were not
>>>>> exercised but I'm not sure how to check for all the possible usages.
>>>>>
>>>>> Do you have any other suggestions that can help improve confidence in
>>>>> the patch?
>>>>
>>>> Unfortunatelly I don't. I just know there were many subtle assumptions
>>>> all over the place so I am rather careful to not touch the code unless
>>>> really necessary.
>>>>
>>>> That being said, I am not opposing your patch.
>>>
>>> Let me be more specific. I am not opposing your patch but we should
>>> definitely need more reviewers to have a look. I am not seeing any
>>> immediate problems with it but I do not see a large improvements either
>>> (slightly less nightmare doesn't make me sleep all that well ;)). So I
>>> will leave the decisions to others.
>> 
>> I hear you - I'd definitely appreciate more eyes on the code change and
>> description.
>
> I like the change in semantics for the routine.  Like you, I examined all
> callers of huge_pte_offset() and it appears that they will not be impacted
> by your change.
>
> My only concern is that arch specific versions of huge_pte_offset, may
> not (yet) follow the new semantic.  Someone could potentially introduce
> a new huge_pte_offset call and depend on the new 'documented' semantics.
> Yet, an unmodified arch specific version of huge_pte_offset might have
> different semantics.  I have not reviewed all the arch specific instances
> of the routine to know if this is even possible.  Just curious if you
> examined these, or perhaps you think this is not an issue?

>From checking through the implementations of huge_pte_offset()
architectures, the change shouldn't break anything. (I also cc'd the
posting to linux-arch for architecture maintainers to take more notice).

This is because existing users actively deal with the different returned
values (NULL, huge pte_t*, swap pte_t*) and are not checking explicitly
for pmd or pud.

Guarding against future users is more tricky - it would definitely help
to align all the implementations.

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[PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

[Punit Agrawal]

When walking the page tables to resolve an address that points to
!p*d_present() entry, huge_pte_offset() returns inconsistent values
depending on the level of page table (PUD or PMD).

It returns NULL in the case of a PUD entry while in the case of a PMD
entry, it returns a pointer to the page table entry.

A similar inconsitency exists when handling swap entries - returns NULL
for a PUD entry while a pointer to the pte_t is retured for the PMD entry.

Update huge_pte_offset() to make the behaviour consistent - return a
pointer to the pte_t for hugepage or swap entries. Only return NULL in
instances where we have a p*d_none() entry and the size parameter
doesn't match the hugepage size at this level of the page table.

Document the behaviour to clarify the expected behaviour of this function.
This is to set clear semantics for architecture specific implementations
of huge_pte_offset().

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Steve Capper <steve.capper@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
---

Hi Andrew,

>From discussions on the arm64 implementation of huge_pte_offset()[0]
we realised that there is benefit from returning a pte_t* in the case
of p*d_none().

The fault handling code in hugetlb_fault() can handle p*d_none()
entries and saves an extra round trip to huge_pte_alloc(). Other
callers of huge_pte_offset() should be ok as well.

Apologies for sending a late update but I thought if we are defining
the semantics, it's worth getting them right.

Could you please pick this version please?

Thanks,
Punit

[0] http://www.spinics.net/lists/linux-mm/msg133699.html

v2: 

 mm/hugetlb.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 31e207cb399b..1d54a131bdd5 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4600,6 +4600,15 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
 	return pte;
 }
 
+/*
+ * huge_pte_offset() - Walk the page table to resolve the hugepage
+ * entry at address @addr
+ *
+ * Return: Pointer to page table or swap entry (PUD or PMD) for
+ * address @addr, or NULL if a p*d_none() entry is encountered and the
+ * size @sz doesn't match the hugepage size at this level of the page
+ * table.
+ */
 pte_t *huge_pte_offset(struct mm_struct *mm,
 		       unsigned long addr, unsigned long sz)
 {
@@ -4614,13 +4623,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
 	p4d = p4d_offset(pgd, addr);
 	if (!p4d_present(*p4d))
 		return NULL;
+
 	pud = pud_offset(p4d, addr);
-	if (!pud_present(*pud))
+	if (sz != PUD_SIZE && pud_none(*pud))
 		return NULL;
-	if (pud_huge(*pud))
+	/* hugepage or swap? */
+	if (pud_huge(*pud) || !pud_present(*pud))
 		return (pte_t *)pud;
+
 	pmd = pmd_offset(pud, addr);
-	return (pte_t *) pmd;
+	if (sz != PMD_SIZE && pmd_none(*pmd))
+		return NULL;
+	/* hugepage or swap? */
+	if (pmd_huge(*pmd) || !pmd_present(*pmd))
+		return (pte_t *)pmd;
+
+	return NULL;
 }
 
 #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
-- 
2.13.2

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Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

[Mike Kravetz]

On 08/18/2017 07:54 AM, Punit Agrawal wrote:
> When walking the page tables to resolve an address that points to
> !p*d_present() entry, huge_pte_offset() returns inconsistent values
> depending on the level of page table (PUD or PMD).
> 
> It returns NULL in the case of a PUD entry while in the case of a PMD
> entry, it returns a pointer to the page table entry.
> 
> A similar inconsitency exists when handling swap entries - returns NULL
> for a PUD entry while a pointer to the pte_t is retured for the PMD entry.
> 
> Update huge_pte_offset() to make the behaviour consistent - return a
> pointer to the pte_t for hugepage or swap entries. Only return NULL in
> instances where we have a p*d_none() entry and the size parameter
> doesn't match the hugepage size at this level of the page table.
> 
> Document the behaviour to clarify the expected behaviour of this function.
> This is to set clear semantics for architecture specific implementations
> of huge_pte_offset().
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Steve Capper <steve.capper@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> ---
> 
> Hi Andrew,
> 
> From discussions on the arm64 implementation of huge_pte_offset()[0]
> we realised that there is benefit from returning a pte_t* in the case
> of p*d_none().
> 
> The fault handling code in hugetlb_fault() can handle p*d_none()
> entries and saves an extra round trip to huge_pte_alloc(). Other
> callers of huge_pte_offset() should be ok as well.

Yes, this change would eliminate that call to huge_pte_alloc() in
hugetlb_fault().  However, huge_pte_offset() is now returning a pointer
to a p*d_none() pte in some instances where it would have previously
returned NULL.  Correct?

I went through the callers, and like you am fairly confident that they
can handle this situation.  But, returning  p*d_none() instead of NULL
does change the execution path in several routines such as
copy_hugetlb_page_range, __unmap_hugepage_range hugetlb_change_protection,
and follow_hugetlb_page.  If huge_pte_alloc() returns NULL to these
routines, they do a quick continue, exit, etc.  If they are returned
a pointer, they typically lock the page table(s) and then check for
p*d_none() before continuing, exiting, etc.  So, it appears that these
routines could potentially slow down a bit with this change (in the specific
case of p*d_none).

I 'think' one could argue that the the fault case is more important.  So,
the savings there would outweigh any potential slowdown in the other
routines.

IMO, this new version of the patch has more potential for issues than
the previous version.  It would be helpful if others could take a look.

One thing I am still 'thinking' about is how this patch could potentially
change behavior in huge_pmd_share.  With the patch, pmd sharing could
potentially be set up in situations (pmd_none) where it previously would
not have been set up.  I don't think this is an issue, but any changes to
this concerns me.

-- 
Mike Kravetz

> 
> Apologies for sending a late update but I thought if we are defining
> the semantics, it's worth getting them right.
> 
> Could you please pick this version please?
> 
> Thanks,
> Punit
> 
> [0] http://www.spinics.net/lists/linux-mm/msg133699.html
> 
> v2: 
> 
>  mm/hugetlb.c | 24 +++++++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 31e207cb399b..1d54a131bdd5 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4600,6 +4600,15 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>  	return pte;
>  }
>  
> +/*
> + * huge_pte_offset() - Walk the page table to resolve the hugepage
> + * entry at address @addr
> + *
> + * Return: Pointer to page table or swap entry (PUD or PMD) for
> + * address @addr, or NULL if a p*d_none() entry is encountered and the
> + * size @sz doesn't match the hugepage size at this level of the page
> + * table.
> + */
>  pte_t *huge_pte_offset(struct mm_struct *mm,
>  		       unsigned long addr, unsigned long sz)
>  {
> @@ -4614,13 +4623,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>  	p4d = p4d_offset(pgd, addr);
>  	if (!p4d_present(*p4d))
>  		return NULL;
> +
>  	pud = pud_offset(p4d, addr);
> -	if (!pud_present(*pud))
> +	if (sz != PUD_SIZE && pud_none(*pud))
>  		return NULL;
> -	if (pud_huge(*pud))
> +	/* hugepage or swap? */
> +	if (pud_huge(*pud) || !pud_present(*pud))
>  		return (pte_t *)pud;
> +
>  	pmd = pmd_offset(pud, addr);
> -	return (pte_t *) pmd;
> +	if (sz != PMD_SIZE && pmd_none(*pmd))
> +		return NULL;
> +	/* hugepage or swap? */
> +	if (pmd_huge(*pmd) || !pmd_present(*pmd))
> +		return (pte_t *)pmd;
> +
> +	return NULL;
>  }
>  
>  #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
> 

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Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

[Catalin Marinas]

On Fri, Aug 18, 2017 at 02:29:18PM -0700, Mike Kravetz wrote:
> On 08/18/2017 07:54 AM, Punit Agrawal wrote:
> > When walking the page tables to resolve an address that points to
> > !p*d_present() entry, huge_pte_offset() returns inconsistent values
> > depending on the level of page table (PUD or PMD).
> > 
> > It returns NULL in the case of a PUD entry while in the case of a PMD
> > entry, it returns a pointer to the page table entry.
> > 
> > A similar inconsitency exists when handling swap entries - returns NULL
> > for a PUD entry while a pointer to the pte_t is retured for the PMD entry.
> > 
> > Update huge_pte_offset() to make the behaviour consistent - return a
> > pointer to the pte_t for hugepage or swap entries. Only return NULL in
> > instances where we have a p*d_none() entry and the size parameter
> > doesn't match the hugepage size at this level of the page table.
> > 
> > Document the behaviour to clarify the expected behaviour of this function.
> > This is to set clear semantics for architecture specific implementations
> > of huge_pte_offset().
> > 
> > Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Cc: Steve Capper <steve.capper@arm.com>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Mike Kravetz <mike.kravetz@oracle.com>
> > ---
> > 
> > Hi Andrew,
> > 
> > From discussions on the arm64 implementation of huge_pte_offset()[0]
> > we realised that there is benefit from returning a pte_t* in the case
> > of p*d_none().
> > 
> > The fault handling code in hugetlb_fault() can handle p*d_none()
> > entries and saves an extra round trip to huge_pte_alloc(). Other
> > callers of huge_pte_offset() should be ok as well.
> 
> Yes, this change would eliminate that call to huge_pte_alloc() in
> hugetlb_fault().  However, huge_pte_offset() is now returning a pointer
> to a p*d_none() pte in some instances where it would have previously
> returned NULL.  Correct?

Yes (whether it was previously the right thing to return is a different
matter; that's what we are trying to clarify in the generic code so that
we can have similar semantics on arm64).

> I went through the callers, and like you am fairly confident that they
> can handle this situation.  But, returning  p*d_none() instead of NULL
> does change the execution path in several routines such as
> copy_hugetlb_page_range, __unmap_hugepage_range hugetlb_change_protection,
> and follow_hugetlb_page.  If huge_pte_alloc() returns NULL to these
> routines, they do a quick continue, exit, etc.  If they are returned
> a pointer, they typically lock the page table(s) and then check for
> p*d_none() before continuing, exiting, etc.  So, it appears that these
> routines could potentially slow down a bit with this change (in the specific
> case of p*d_none).

Arguably (well, my interpretation), it should return a NULL only if the
entry is a table entry, potentially pointing to a next level (pmd). In
the pud case, this means that sz < PUD_SIZE.

If the pud is a last level huge page entry (either present or !present),
huge_pte_offset() should return the pointer to it and never NULL. If the
entry is a swap or migration one (pte_present() == false) with the
current code we don't even enter the corresponding checks in
copy_hugetlb_page_range().

I also assume that the ptl __unmap_hugepage_range() is taken to avoid
some race when the entry is a huge page (present or not). If such race
doesn't exist, we could as well check the huge_pte_none() outside the
locked region (which is what the current huge_pte_offset() does with
!pud_present()).

IMHO, while the current generic huge_pte_offset() avoids some code paths
in the functions you mentioned, the results are not always correct
(missing swap/migration entries or potentially racy).

-- 
Catalin

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Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

[Mike Kravetz]

On 08/21/2017 11:07 AM, Catalin Marinas wrote:
> On Fri, Aug 18, 2017 at 02:29:18PM -0700, Mike Kravetz wrote:
>> On 08/18/2017 07:54 AM, Punit Agrawal wrote:
>>> When walking the page tables to resolve an address that points to
>>> !p*d_present() entry, huge_pte_offset() returns inconsistent values
>>> depending on the level of page table (PUD or PMD).
>>>
>>> It returns NULL in the case of a PUD entry while in the case of a PMD
>>> entry, it returns a pointer to the page table entry.
>>>
>>> A similar inconsitency exists when handling swap entries - returns NULL
>>> for a PUD entry while a pointer to the pte_t is retured for the PMD entry.
>>>
>>> Update huge_pte_offset() to make the behaviour consistent - return a
>>> pointer to the pte_t for hugepage or swap entries. Only return NULL in
>>> instances where we have a p*d_none() entry and the size parameter
>>> doesn't match the hugepage size at this level of the page table.
>>>
>>> Document the behaviour to clarify the expected behaviour of this function.
>>> This is to set clear semantics for architecture specific implementations
>>> of huge_pte_offset().
>>>
>>> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>> Cc: Steve Capper <steve.capper@arm.com>
>>> Cc: Will Deacon <will.deacon@arm.com>
>>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>>> ---
>>>
>>> Hi Andrew,
>>>
>>> From discussions on the arm64 implementation of huge_pte_offset()[0]
>>> we realised that there is benefit from returning a pte_t* in the case
>>> of p*d_none().
>>>
>>> The fault handling code in hugetlb_fault() can handle p*d_none()
>>> entries and saves an extra round trip to huge_pte_alloc(). Other
>>> callers of huge_pte_offset() should be ok as well.
>>
>> Yes, this change would eliminate that call to huge_pte_alloc() in
>> hugetlb_fault().  However, huge_pte_offset() is now returning a pointer
>> to a p*d_none() pte in some instances where it would have previously
>> returned NULL.  Correct?
> 
> Yes (whether it was previously the right thing to return is a different
> matter; that's what we are trying to clarify in the generic code so that
> we can have similar semantics on arm64).
> 
>> I went through the callers, and like you am fairly confident that they
>> can handle this situation.  But, returning  p*d_none() instead of NULL
>> does change the execution path in several routines such as
>> copy_hugetlb_page_range, __unmap_hugepage_range hugetlb_change_protection,
>> and follow_hugetlb_page.  If huge_pte_alloc() returns NULL to these
>> routines, they do a quick continue, exit, etc.  If they are returned
>> a pointer, they typically lock the page table(s) and then check for
>> p*d_none() before continuing, exiting, etc.  So, it appears that these
>> routines could potentially slow down a bit with this change (in the specific
>> case of p*d_none).
> 
> Arguably (well, my interpretation), it should return a NULL only if the
> entry is a table entry, potentially pointing to a next level (pmd). In
> the pud case, this means that sz < PUD_SIZE.
> 
> If the pud is a last level huge page entry (either present or !present),
> huge_pte_offset() should return the pointer to it and never NULL. If the
> entry is a swap or migration one (pte_present() == false) with the
> current code we don't even enter the corresponding checks in
> copy_hugetlb_page_range().
> 
> I also assume that the ptl __unmap_hugepage_range() is taken to avoid
> some race when the entry is a huge page (present or not). If such race
> doesn't exist, we could as well check the huge_pte_none() outside the
> locked region (which is what the current huge_pte_offset() does with
> !pud_present()).
> 
> IMHO, while the current generic huge_pte_offset() avoids some code paths
> in the functions you mentioned, the results are not always correct
> (missing swap/migration entries or potentially racy).

Thanks Catalin,

The more I look at this code and think about it, the more I like it.  As
Michal previously mentioned, changes in this area can break things in subtle
ways.  That is why I was cautious and asked for more people to look at it.
My primary concerns with these changes in this area were:
- Any potential changes in behavior.  I think this has been sufficiently
  explored.  While there may be small differences in behavior (for the
  better), this change should not introduce any bugs/breakage.
- Other arch specific implementations are not aligned with the new
  behavior.  Again, this should not cause any issues.  Punit (and I) have
  looked at the arch specific implementations for issues and found none.
  In addition, since we are not changing any of the 'calling code', no
  issues should be introduced for arch specific implementations.

I like the new semantics and did not find any issues.

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

-- 
Mike Kravetz

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Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

[Catalin Marinas]

On Fri, Aug 18, 2017 at 03:54:15PM +0100, Punit Agrawal wrote:
> When walking the page tables to resolve an address that points to
> !p*d_present() entry, huge_pte_offset() returns inconsistent values
> depending on the level of page table (PUD or PMD).
> 
> It returns NULL in the case of a PUD entry while in the case of a PMD
> entry, it returns a pointer to the page table entry.
> 
> A similar inconsitency exists when handling swap entries - returns NULL
> for a PUD entry while a pointer to the pte_t is retured for the PMD entry.
> 
> Update huge_pte_offset() to make the behaviour consistent - return a
> pointer to the pte_t for hugepage or swap entries. Only return NULL in
> instances where we have a p*d_none() entry and the size parameter
> doesn't match the hugepage size at this level of the page table.
> 
> Document the behaviour to clarify the expected behaviour of this function.
> This is to set clear semantics for architecture specific implementations
> of huge_pte_offset().
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Steve Capper <steve.capper@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>

FWIW:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Thanks.

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Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

[Punit Agrawal]

Hi Mike,

Mike Kravetz <mike.kravetz@oracle.com> writes:

> On 08/21/2017 11:07 AM, Catalin Marinas wrote:
>> On Fri, Aug 18, 2017 at 02:29:18PM -0700, Mike Kravetz wrote:
>>> On 08/18/2017 07:54 AM, Punit Agrawal wrote:
>>>> When walking the page tables to resolve an address that points to
>>>> !p*d_present() entry, huge_pte_offset() returns inconsistent values
>>>> depending on the level of page table (PUD or PMD).
>>>>
>>>> It returns NULL in the case of a PUD entry while in the case of a PMD
>>>> entry, it returns a pointer to the page table entry.
>>>>
>>>> A similar inconsitency exists when handling swap entries - returns NULL
>>>> for a PUD entry while a pointer to the pte_t is retured for the PMD entry.
>>>>
>>>> Update huge_pte_offset() to make the behaviour consistent - return a
>>>> pointer to the pte_t for hugepage or swap entries. Only return NULL in
>>>> instances where we have a p*d_none() entry and the size parameter
>>>> doesn't match the hugepage size at this level of the page table.
>>>>
>>>> Document the behaviour to clarify the expected behaviour of this function.
>>>> This is to set clear semantics for architecture specific implementations
>>>> of huge_pte_offset().
>>>>
>>>> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>>> Cc: Steve Capper <steve.capper@arm.com>
>>>> Cc: Will Deacon <will.deacon@arm.com>
>>>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>>>> ---
>>>>
>>>> Hi Andrew,
>>>>
>>>> From discussions on the arm64 implementation of huge_pte_offset()[0]
>>>> we realised that there is benefit from returning a pte_t* in the case
>>>> of p*d_none().
>>>>
>>>> The fault handling code in hugetlb_fault() can handle p*d_none()
>>>> entries and saves an extra round trip to huge_pte_alloc(). Other
>>>> callers of huge_pte_offset() should be ok as well.
>>>
>>> Yes, this change would eliminate that call to huge_pte_alloc() in
>>> hugetlb_fault().  However, huge_pte_offset() is now returning a pointer
>>> to a p*d_none() pte in some instances where it would have previously
>>> returned NULL.  Correct?
>> 
>> Yes (whether it was previously the right thing to return is a different
>> matter; that's what we are trying to clarify in the generic code so that
>> we can have similar semantics on arm64).
>> 
>>> I went through the callers, and like you am fairly confident that they
>>> can handle this situation.  But, returning  p*d_none() instead of NULL
>>> does change the execution path in several routines such as
>>> copy_hugetlb_page_range, __unmap_hugepage_range hugetlb_change_protection,
>>> and follow_hugetlb_page.  If huge_pte_alloc() returns NULL to these
>>> routines, they do a quick continue, exit, etc.  If they are returned
>>> a pointer, they typically lock the page table(s) and then check for
>>> p*d_none() before continuing, exiting, etc.  So, it appears that these
>>> routines could potentially slow down a bit with this change (in the specific
>>> case of p*d_none).
>> 
>> Arguably (well, my interpretation), it should return a NULL only if the
>> entry is a table entry, potentially pointing to a next level (pmd). In
>> the pud case, this means that sz < PUD_SIZE.
>> 
>> If the pud is a last level huge page entry (either present or !present),
>> huge_pte_offset() should return the pointer to it and never NULL. If the
>> entry is a swap or migration one (pte_present() == false) with the
>> current code we don't even enter the corresponding checks in
>> copy_hugetlb_page_range().
>> 
>> I also assume that the ptl __unmap_hugepage_range() is taken to avoid
>> some race when the entry is a huge page (present or not). If such race
>> doesn't exist, we could as well check the huge_pte_none() outside the
>> locked region (which is what the current huge_pte_offset() does with
>> !pud_present()).
>> 
>> IMHO, while the current generic huge_pte_offset() avoids some code paths
>> in the functions you mentioned, the results are not always correct
>> (missing swap/migration entries or potentially racy).
>
> Thanks Catalin,
>
> The more I look at this code and think about it, the more I like it.  As
> Michal previously mentioned, changes in this area can break things in subtle
> ways.  That is why I was cautious and asked for more people to look at it.
> My primary concerns with these changes in this area were:
> - Any potential changes in behavior.  I think this has been sufficiently
>   explored.  While there may be small differences in behavior (for the
>   better), this change should not introduce any bugs/breakage.
> - Other arch specific implementations are not aligned with the new
>   behavior.  Again, this should not cause any issues.  Punit (and I) have
>   looked at the arch specific implementations for issues and found none.
>   In addition, since we are not changing any of the 'calling code', no
>   issues should be introduced for arch specific implementations.
>
> I like the new semantics and did not find any issues.
>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

Thanks for reviewing the updated semantics against existing usage. I'll
monitor the lists for any reported breakage but please do shout out if
you notice any issues.

Thanks,
Punit

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Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

[Michal Hocko]

On Fri 18-08-17 15:54:15, Punit Agrawal wrote:
> When walking the page tables to resolve an address that points to
> !p*d_present() entry, huge_pte_offset() returns inconsistent values
> depending on the level of page table (PUD or PMD).
> 
> It returns NULL in the case of a PUD entry while in the case of a PMD
> entry, it returns a pointer to the page table entry.
> 
> A similar inconsitency exists when handling swap entries - returns NULL
> for a PUD entry while a pointer to the pte_t is retured for the PMD entry.
> 
> Update huge_pte_offset() to make the behaviour consistent - return a
> pointer to the pte_t for hugepage or swap entries. Only return NULL in
> instances where we have a p*d_none() entry and the size parameter
> doesn't match the hugepage size at this level of the page table.
> 
> Document the behaviour to clarify the expected behaviour of this function.
> This is to set clear semantics for architecture specific implementations
> of huge_pte_offset().
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Steve Capper <steve.capper@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>

I always thought that the weird semantic is a result of the hugetlb pte
sharing. But now that I dug into history it has been added by
02b0ccef903e ("[PATCH] hugetlb: check p?d_present in huge_pte_offset()")
for a completely different reason. I suspec the weird semantic just
wasn't noticed back then.

Anyway, I didn't find any problem with the patch
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 
> Hi Andrew,
> 
> >From discussions on the arm64 implementation of huge_pte_offset()[0]
> we realised that there is benefit from returning a pte_t* in the case
> of p*d_none().
> 
> The fault handling code in hugetlb_fault() can handle p*d_none()
> entries and saves an extra round trip to huge_pte_alloc(). Other
> callers of huge_pte_offset() should be ok as well.
> 
> Apologies for sending a late update but I thought if we are defining
> the semantics, it's worth getting them right.
> 
> Could you please pick this version please?
> 
> Thanks,
> Punit
> 
> [0] http://www.spinics.net/lists/linux-mm/msg133699.html
> 
> v2: 
> 
>  mm/hugetlb.c | 24 +++++++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 31e207cb399b..1d54a131bdd5 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4600,6 +4600,15 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>  	return pte;
>  }
>  
> +/*
> + * huge_pte_offset() - Walk the page table to resolve the hugepage
> + * entry at address @addr
> + *
> + * Return: Pointer to page table or swap entry (PUD or PMD) for
> + * address @addr, or NULL if a p*d_none() entry is encountered and the
> + * size @sz doesn't match the hugepage size at this level of the page
> + * table.
> + */
>  pte_t *huge_pte_offset(struct mm_struct *mm,
>  		       unsigned long addr, unsigned long sz)
>  {
> @@ -4614,13 +4623,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>  	p4d = p4d_offset(pgd, addr);
>  	if (!p4d_present(*p4d))
>  		return NULL;
> +
>  	pud = pud_offset(p4d, addr);
> -	if (!pud_present(*pud))
> +	if (sz != PUD_SIZE && pud_none(*pud))
>  		return NULL;
> -	if (pud_huge(*pud))
> +	/* hugepage or swap? */
> +	if (pud_huge(*pud) || !pud_present(*pud))
>  		return (pte_t *)pud;
> +
>  	pmd = pmd_offset(pud, addr);
> -	return (pte_t *) pmd;
> +	if (sz != PMD_SIZE && pmd_none(*pmd))
> +		return NULL;
> +	/* hugepage or swap? */
> +	if (pmd_huge(*pmd) || !pmd_present(*pmd))
> +		return (pte_t *)pmd;
> +
> +	return NULL;
>  }
>  
>  #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
> -- 
> 2.13.2
> 

-- 
Michal Hocko
SUSE Labs

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