diff mbox series

[v1,2/2] mm: remove redundant smp_wmb()

Message ID 20210828042306.42886-3-zhengqi.arch@bytedance.com (mailing list archive)
State New
Headers show
Series Do some code cleanups related to mm | expand

Commit Message

Qi Zheng Aug. 28, 2021, 4:23 a.m. UTC
The smp_wmb() which is in the __pte_alloc() is used to
ensure all ptes setup is visible before the pte is made
visible to other CPUs by being put into page tables. We
only need this when the pte is actually populated, so
move it to pte_install(). __pte_alloc_kernel(),
__p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar
to this case.

We can also defer smp_wmb() to the place where the pmd entry
is really populated by preallocated pte. There are two kinds
of user of preallocated pte, one is filemap & finish_fault(),
another is THP. The former does not need another smp_wmb()
because the smp_wmb() has been done by pte_install().
Fortunately, the latter also does not need another smp_wmb()
because there is already a smp_wmb() before populating the
new pte when the THP uses a preallocated pte to split a huge
pmd.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/memory.c         | 47 ++++++++++++++++++++---------------------------
 mm/sparse-vmemmap.c |  2 +-
 2 files changed, 21 insertions(+), 28 deletions(-)

Comments

David Hildenbrand Aug. 31, 2021, 10:02 a.m. UTC | #1
On 28.08.21 06:23, Qi Zheng wrote:
> The smp_wmb() which is in the __pte_alloc() is used to
> ensure all ptes setup is visible before the pte is made
> visible to other CPUs by being put into page tables. We
> only need this when the pte is actually populated, so
> move it to pte_install(). __pte_alloc_kernel(),
> __p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar
> to this case.
> 
> We can also defer smp_wmb() to the place where the pmd entry
> is really populated by preallocated pte. There are two kinds
> of user of preallocated pte, one is filemap & finish_fault(),
> another is THP. The former does not need another smp_wmb()
> because the smp_wmb() has been done by pte_install().
> Fortunately, the latter also does not need another smp_wmb()
> because there is already a smp_wmb() before populating the
> new pte when the THP uses a preallocated pte to split a huge
> pmd.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> ---
>   mm/memory.c         | 47 ++++++++++++++++++++---------------------------
>   mm/sparse-vmemmap.c |  2 +-
>   2 files changed, 21 insertions(+), 28 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index ef7b1762e996..9c7534187454 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -439,6 +439,20 @@ void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte)
>   
>   	if (likely(pmd_none(*pmd))) {	/* Has another populated it ? */
>   		mm_inc_nr_ptes(mm);
> +		/*
> +		 * Ensure all pte setup (eg. pte page lock and page clearing) are
> +		 * visible before the pte is made visible to other CPUs by being
> +		 * put into page tables.
> +		 *
> +		 * The other side of the story is the pointer chasing in the page
> +		 * table walking code (when walking the page table without locking;
> +		 * ie. most of the time). Fortunately, these data accesses consist
> +		 * of a chain of data-dependent loads, meaning most CPUs (alpha
> +		 * being the notable exception) will already guarantee loads are
> +		 * seen in-order. See the alpha page table accessors for the
> +		 * smp_rmb() barriers in page table walking code.
> +		 */
> +		smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
>   		pmd_populate(mm, pmd, *pte);
>   		*pte = NULL;
>   	}
> @@ -451,21 +465,6 @@ int __pte_alloc(struct mm_struct *mm, pmd_t *pmd)
>   	if (!new)
>   		return -ENOMEM;
>   
> -	/*
> -	 * Ensure all pte setup (eg. pte page lock and page clearing) are
> -	 * visible before the pte is made visible to other CPUs by being
> -	 * put into page tables.
> -	 *
> -	 * The other side of the story is the pointer chasing in the page
> -	 * table walking code (when walking the page table without locking;
> -	 * ie. most of the time). Fortunately, these data accesses consist
> -	 * of a chain of data-dependent loads, meaning most CPUs (alpha
> -	 * being the notable exception) will already guarantee loads are
> -	 * seen in-order. See the alpha page table accessors for the
> -	 * smp_rmb() barriers in page table walking code.
> -	 */
> -	smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
> -
>   	pmd_install(mm, pmd, &new);
>   	if (new)
>   		pte_free(mm, new);
> @@ -478,10 +477,9 @@ int __pte_alloc_kernel(pmd_t *pmd)
>   	if (!new)
>   		return -ENOMEM;
>   
> -	smp_wmb(); /* See comment in __pte_alloc */
> -
>   	spin_lock(&init_mm.page_table_lock);
>   	if (likely(pmd_none(*pmd))) {	/* Has another populated it ? */
> +		smp_wmb(); /* See comment in pmd_install() */
>   		pmd_populate_kernel(&init_mm, pmd, new);
>   		new = NULL;
>   	}
> @@ -3857,7 +3855,6 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
>   		vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
>   		if (!vmf->prealloc_pte)
>   			return VM_FAULT_OOM;
> -		smp_wmb(); /* See comment in __pte_alloc() */
>   	}
>   
>   	ret = vma->vm_ops->fault(vmf);
> @@ -3919,7 +3916,6 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
>   		vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
>   		if (!vmf->prealloc_pte)
>   			return VM_FAULT_OOM;
> -		smp_wmb(); /* See comment in __pte_alloc() */
>   	}
>   
>   	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> @@ -4144,7 +4140,6 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
>   		vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm);
>   		if (!vmf->prealloc_pte)
>   			return VM_FAULT_OOM;
> -		smp_wmb(); /* See comment in __pte_alloc() */
>   	}
>   
>   	return vmf->vma->vm_ops->map_pages(vmf, start_pgoff, end_pgoff);
> @@ -4819,13 +4814,13 @@ int __p4d_alloc(struct mm_struct *mm, pgd_t *pgd, unsigned long address)
>   	if (!new)
>   		return -ENOMEM;
>   
> -	smp_wmb(); /* See comment in __pte_alloc */
> -
>   	spin_lock(&mm->page_table_lock);
>   	if (pgd_present(*pgd))		/* Another has populated it */
>   		p4d_free(mm, new);
> -	else
> +	else {
> +		smp_wmb(); /* See comment in pmd_install() */
>   		pgd_populate(mm, pgd, new);
> +	}

Nit:

if () {

} else {

}

see Documentation/process/coding-style.rst

"This does not apply if only one branch of a conditional statement is a 
single statement; in the latter case use braces in both branches:"


Apart from that, I think this is fine,

Acked-by: David Hildenbrand <david@redhat.com>
Vlastimil Babka Aug. 31, 2021, 10:20 a.m. UTC | #2
On 8/28/21 06:23, Qi Zheng wrote:
> The smp_wmb() which is in the __pte_alloc() is used to
> ensure all ptes setup is visible before the pte is made
> visible to other CPUs by being put into page tables. We
> only need this when the pte is actually populated, so
> move it to pte_install(). __pte_alloc_kernel(),

It's named pmd_install()?

> __p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar
> to this case.
> 
> We can also defer smp_wmb() to the place where the pmd entry
> is really populated by preallocated pte. There are two kinds
> of user of preallocated pte, one is filemap & finish_fault(),
> another is THP. The former does not need another smp_wmb()
> because the smp_wmb() has been done by pte_install().

Same here.

> Fortunately, the latter also does not need another smp_wmb()
> because there is already a smp_wmb() before populating the
> new pte when the THP uses a preallocated pte to split a huge
> pmd.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/memory.c         | 47 ++++++++++++++++++++---------------------------
>  mm/sparse-vmemmap.c |  2 +-
>  2 files changed, 21 insertions(+), 28 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index ef7b1762e996..9c7534187454 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -439,6 +439,20 @@ void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte)
>  
>  	if (likely(pmd_none(*pmd))) {	/* Has another populated it ? */
>  		mm_inc_nr_ptes(mm);
> +		/*
> +		 * Ensure all pte setup (eg. pte page lock and page clearing) are
> +		 * visible before the pte is made visible to other CPUs by being
> +		 * put into page tables.
> +		 *
> +		 * The other side of the story is the pointer chasing in the page
> +		 * table walking code (when walking the page table without locking;
> +		 * ie. most of the time). Fortunately, these data accesses consist
> +		 * of a chain of data-dependent loads, meaning most CPUs (alpha
> +		 * being the notable exception) will already guarantee loads are
> +		 * seen in-order. See the alpha page table accessors for the
> +		 * smp_rmb() barriers in page table walking code.
> +		 */
> +		smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */

So, could it? :)
Qi Zheng Aug. 31, 2021, 12:36 p.m. UTC | #3
On 2021/8/31 PM6:02, David Hildenbrand wrote:
> On 28.08.21 06:23, Qi Zheng wrote:
>> The smp_wmb() which is in the __pte_alloc() is used to
>> ensure all ptes setup is visible before the pte is made
>> visible to other CPUs by being put into page tables. We
>> only need this when the pte is actually populated, so
>> move it to pte_install(). __pte_alloc_kernel(),
>> __p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar
>> to this case.
>>
>> We can also defer smp_wmb() to the place where the pmd entry
>> is really populated by preallocated pte. There are two kinds
>> of user of preallocated pte, one is filemap & finish_fault(),
>> another is THP. The former does not need another smp_wmb()
>> because the smp_wmb() has been done by pte_install().
>> Fortunately, the latter also does not need another smp_wmb()
>> because there is already a smp_wmb() before populating the
>> new pte when the THP uses a preallocated pte to split a huge
>> pmd.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>   mm/memory.c         | 47 
>> ++++++++++++++++++++---------------------------
>>   mm/sparse-vmemmap.c |  2 +-
>>   2 files changed, 21 insertions(+), 28 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index ef7b1762e996..9c7534187454 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -439,6 +439,20 @@ void pmd_install(struct mm_struct *mm, pmd_t 
>> *pmd, pgtable_t *pte)
>>       if (likely(pmd_none(*pmd))) {    /* Has another populated it ? */
>>           mm_inc_nr_ptes(mm);
>> +        /*
>> +         * Ensure all pte setup (eg. pte page lock and page clearing) 
>> are
>> +         * visible before the pte is made visible to other CPUs by being
>> +         * put into page tables.
>> +         *
>> +         * The other side of the story is the pointer chasing in the 
>> page
>> +         * table walking code (when walking the page table without 
>> locking;
>> +         * ie. most of the time). Fortunately, these data accesses 
>> consist
>> +         * of a chain of data-dependent loads, meaning most CPUs (alpha
>> +         * being the notable exception) will already guarantee loads are
>> +         * seen in-order. See the alpha page table accessors for the
>> +         * smp_rmb() barriers in page table walking code.
>> +         */
>> +        smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
>>           pmd_populate(mm, pmd, *pte);
>>           *pte = NULL;
>>       }
>> @@ -451,21 +465,6 @@ int __pte_alloc(struct mm_struct *mm, pmd_t *pmd)
>>       if (!new)
>>           return -ENOMEM;
>> -    /*
>> -     * Ensure all pte setup (eg. pte page lock and page clearing) are
>> -     * visible before the pte is made visible to other CPUs by being
>> -     * put into page tables.
>> -     *
>> -     * The other side of the story is the pointer chasing in the page
>> -     * table walking code (when walking the page table without locking;
>> -     * ie. most of the time). Fortunately, these data accesses consist
>> -     * of a chain of data-dependent loads, meaning most CPUs (alpha
>> -     * being the notable exception) will already guarantee loads are
>> -     * seen in-order. See the alpha page table accessors for the
>> -     * smp_rmb() barriers in page table walking code.
>> -     */
>> -    smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
>> -
>>       pmd_install(mm, pmd, &new);
>>       if (new)
>>           pte_free(mm, new);
>> @@ -478,10 +477,9 @@ int __pte_alloc_kernel(pmd_t *pmd)
>>       if (!new)
>>           return -ENOMEM;
>> -    smp_wmb(); /* See comment in __pte_alloc */
>> -
>>       spin_lock(&init_mm.page_table_lock);
>>       if (likely(pmd_none(*pmd))) {    /* Has another populated it ? */
>> +        smp_wmb(); /* See comment in pmd_install() */
>>           pmd_populate_kernel(&init_mm, pmd, new);
>>           new = NULL;
>>       }
>> @@ -3857,7 +3855,6 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
>>           vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
>>           if (!vmf->prealloc_pte)
>>               return VM_FAULT_OOM;
>> -        smp_wmb(); /* See comment in __pte_alloc() */
>>       }
>>       ret = vma->vm_ops->fault(vmf);
>> @@ -3919,7 +3916,6 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, 
>> struct page *page)
>>           vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
>>           if (!vmf->prealloc_pte)
>>               return VM_FAULT_OOM;
>> -        smp_wmb(); /* See comment in __pte_alloc() */
>>       }
>>       vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>> @@ -4144,7 +4140,6 @@ static vm_fault_t do_fault_around(struct 
>> vm_fault *vmf)
>>           vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm);
>>           if (!vmf->prealloc_pte)
>>               return VM_FAULT_OOM;
>> -        smp_wmb(); /* See comment in __pte_alloc() */
>>       }
>>       return vmf->vma->vm_ops->map_pages(vmf, start_pgoff, end_pgoff);
>> @@ -4819,13 +4814,13 @@ int __p4d_alloc(struct mm_struct *mm, pgd_t 
>> *pgd, unsigned long address)
>>       if (!new)
>>           return -ENOMEM;
>> -    smp_wmb(); /* See comment in __pte_alloc */
>> -
>>       spin_lock(&mm->page_table_lock);
>>       if (pgd_present(*pgd))        /* Another has populated it */
>>           p4d_free(mm, new);
>> -    else
>> +    else {
>> +        smp_wmb(); /* See comment in pmd_install() */
>>           pgd_populate(mm, pgd, new);
>> +    }
> 
> Nit:
> 
> if () {
> 
> } else {
> 
> }
> 
> see Documentation/process/coding-style.rst
> 
> "This does not apply if only one branch of a conditional statement is a 
> single statement; in the latter case use braces in both branches:"

Got it.

> 
> 
> Apart from that, I think this is fine,
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> 

Thanks,
Qi
Qi Zheng Aug. 31, 2021, 12:52 p.m. UTC | #4
On 2021/8/31 PM6:20, Vlastimil Babka wrote:
> On 8/28/21 06:23, Qi Zheng wrote:
>> The smp_wmb() which is in the __pte_alloc() is used to
>> ensure all ptes setup is visible before the pte is made
>> visible to other CPUs by being put into page tables. We
>> only need this when the pte is actually populated, so
>> move it to pte_install(). __pte_alloc_kernel(),
> 
> It's named pmd_install()?

Yes, I will update it in the next version.

> 
>> __p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar
>> to this case.
>>
>> We can also defer smp_wmb() to the place where the pmd entry
>> is really populated by preallocated pte. There are two kinds
>> of user of preallocated pte, one is filemap & finish_fault(),
>> another is THP. The former does not need another smp_wmb()
>> because the smp_wmb() has been done by pte_install().
> 
> Same here.
> 
>> Fortunately, the latter also does not need another smp_wmb()
>> because there is already a smp_wmb() before populating the
>> new pte when the THP uses a preallocated pte to split a huge
>> pmd.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>   mm/memory.c         | 47 ++++++++++++++++++++---------------------------
>>   mm/sparse-vmemmap.c |  2 +-
>>   2 files changed, 21 insertions(+), 28 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index ef7b1762e996..9c7534187454 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -439,6 +439,20 @@ void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte)
>>   
>>   	if (likely(pmd_none(*pmd))) {	/* Has another populated it ? */
>>   		mm_inc_nr_ptes(mm);
>> +		/*
>> +		 * Ensure all pte setup (eg. pte page lock and page clearing) are
>> +		 * visible before the pte is made visible to other CPUs by being
>> +		 * put into page tables.
>> +		 *
>> +		 * The other side of the story is the pointer chasing in the page
>> +		 * table walking code (when walking the page table without locking;
>> +		 * ie. most of the time). Fortunately, these data accesses consist
>> +		 * of a chain of data-dependent loads, meaning most CPUs (alpha
>> +		 * being the notable exception) will already guarantee loads are
>> +		 * seen in-order. See the alpha page table accessors for the
>> +		 * smp_rmb() barriers in page table walking code.
>> +		 */
>> +		smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
> 
> So, could it? :)
> 

Yes, it could, but we don't have smp_wmb__after_spin_lock() now.
diff mbox series

Patch

diff --git a/mm/memory.c b/mm/memory.c
index ef7b1762e996..9c7534187454 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -439,6 +439,20 @@  void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte)
 
 	if (likely(pmd_none(*pmd))) {	/* Has another populated it ? */
 		mm_inc_nr_ptes(mm);
+		/*
+		 * Ensure all pte setup (eg. pte page lock and page clearing) are
+		 * visible before the pte is made visible to other CPUs by being
+		 * put into page tables.
+		 *
+		 * The other side of the story is the pointer chasing in the page
+		 * table walking code (when walking the page table without locking;
+		 * ie. most of the time). Fortunately, these data accesses consist
+		 * of a chain of data-dependent loads, meaning most CPUs (alpha
+		 * being the notable exception) will already guarantee loads are
+		 * seen in-order. See the alpha page table accessors for the
+		 * smp_rmb() barriers in page table walking code.
+		 */
+		smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
 		pmd_populate(mm, pmd, *pte);
 		*pte = NULL;
 	}
@@ -451,21 +465,6 @@  int __pte_alloc(struct mm_struct *mm, pmd_t *pmd)
 	if (!new)
 		return -ENOMEM;
 
-	/*
-	 * Ensure all pte setup (eg. pte page lock and page clearing) are
-	 * visible before the pte is made visible to other CPUs by being
-	 * put into page tables.
-	 *
-	 * The other side of the story is the pointer chasing in the page
-	 * table walking code (when walking the page table without locking;
-	 * ie. most of the time). Fortunately, these data accesses consist
-	 * of a chain of data-dependent loads, meaning most CPUs (alpha
-	 * being the notable exception) will already guarantee loads are
-	 * seen in-order. See the alpha page table accessors for the
-	 * smp_rmb() barriers in page table walking code.
-	 */
-	smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
-
 	pmd_install(mm, pmd, &new);
 	if (new)
 		pte_free(mm, new);
@@ -478,10 +477,9 @@  int __pte_alloc_kernel(pmd_t *pmd)
 	if (!new)
 		return -ENOMEM;
 
-	smp_wmb(); /* See comment in __pte_alloc */
-
 	spin_lock(&init_mm.page_table_lock);
 	if (likely(pmd_none(*pmd))) {	/* Has another populated it ? */
+		smp_wmb(); /* See comment in pmd_install() */
 		pmd_populate_kernel(&init_mm, pmd, new);
 		new = NULL;
 	}
@@ -3857,7 +3855,6 @@  static vm_fault_t __do_fault(struct vm_fault *vmf)
 		vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
 		if (!vmf->prealloc_pte)
 			return VM_FAULT_OOM;
-		smp_wmb(); /* See comment in __pte_alloc() */
 	}
 
 	ret = vma->vm_ops->fault(vmf);
@@ -3919,7 +3916,6 @@  vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
 		vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
 		if (!vmf->prealloc_pte)
 			return VM_FAULT_OOM;
-		smp_wmb(); /* See comment in __pte_alloc() */
 	}
 
 	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
@@ -4144,7 +4140,6 @@  static vm_fault_t do_fault_around(struct vm_fault *vmf)
 		vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm);
 		if (!vmf->prealloc_pte)
 			return VM_FAULT_OOM;
-		smp_wmb(); /* See comment in __pte_alloc() */
 	}
 
 	return vmf->vma->vm_ops->map_pages(vmf, start_pgoff, end_pgoff);
@@ -4819,13 +4814,13 @@  int __p4d_alloc(struct mm_struct *mm, pgd_t *pgd, unsigned long address)
 	if (!new)
 		return -ENOMEM;
 
-	smp_wmb(); /* See comment in __pte_alloc */
-
 	spin_lock(&mm->page_table_lock);
 	if (pgd_present(*pgd))		/* Another has populated it */
 		p4d_free(mm, new);
-	else
+	else {
+		smp_wmb(); /* See comment in pmd_install() */
 		pgd_populate(mm, pgd, new);
+	}
 	spin_unlock(&mm->page_table_lock);
 	return 0;
 }
@@ -4842,11 +4837,10 @@  int __pud_alloc(struct mm_struct *mm, p4d_t *p4d, unsigned long address)
 	if (!new)
 		return -ENOMEM;
 
-	smp_wmb(); /* See comment in __pte_alloc */
-
 	spin_lock(&mm->page_table_lock);
 	if (!p4d_present(*p4d)) {
 		mm_inc_nr_puds(mm);
+		smp_wmb(); /* See comment in pmd_install() */
 		p4d_populate(mm, p4d, new);
 	} else	/* Another has populated it */
 		pud_free(mm, new);
@@ -4867,11 +4861,10 @@  int __pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address)
 	if (!new)
 		return -ENOMEM;
 
-	smp_wmb(); /* See comment in __pte_alloc */
-
 	ptl = pud_lock(mm, pud);
 	if (!pud_present(*pud)) {
 		mm_inc_nr_pmds(mm);
+		smp_wmb(); /* See comment in pmd_install() */
 		pud_populate(mm, pud, new);
 	} else	/* Another has populated it */
 		pmd_free(mm, new);
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index bdce883f9286..db6df27c852a 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -76,7 +76,7 @@  static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
 		set_pte_at(&init_mm, addr, pte, entry);
 	}
 
-	/* Make pte visible before pmd. See comment in __pte_alloc(). */
+	/* Make pte visible before pmd. See comment in pmd_install(). */
 	smp_wmb();
 	pmd_populate_kernel(&init_mm, pmd, pgtable);