Re: [PATCH] mm: use standard page table accessors

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 26/11/2025 à 07:47, Wei Yang a écrit :
> Use standard page table accessors i.e pxdp_get() to get the value of
> pxdp.

Please provide more detail of why you want to do that and how you are 
sure it doesn't break existing implementation.

There was similar tentative in the past already which proved to give 
suboptimal results, see discussion here:

https://lore.kernel.org/all/f40ea8bf-0862-41a7-af19-70bfbd838568@csgroup.eu/

Christophe

> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> ---
>   include/linux/pgtable.h | 2 +-
>   mm/huge_memory.c        | 2 +-
>   mm/memory.c             | 8 ++++----
>   3 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index b13b6f42be3c..a9efd58658bc 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1810,7 +1810,7 @@ static inline int pud_trans_unstable(pud_t *pud)
>   {
>   #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && \
>   	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
> -	pud_t pudval = READ_ONCE(*pud);
> +	pud_t pudval = pudp_get(pud);
>   
>   	if (pud_none(pudval) || pud_trans_huge(pudval))
>   		return 1;
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0d2ac331ccad..dd3577e40d16 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1486,7 +1486,7 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>   		}
>   		vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>   		ret = 0;
> -		if (pmd_none(*vmf->pmd)) {
> +		if (pmd_none(pmdp_get(vmf->pmd))) {
>   			ret = check_stable_address_space(vma->vm_mm);
>   			if (ret) {
>   				spin_unlock(vmf->ptl);
> diff --git a/mm/memory.c b/mm/memory.c
> index 8933069948e5..39839bf0c3f5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6193,7 +6193,7 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>   {
>   	pte_t entry;
>   
> -	if (unlikely(pmd_none(*vmf->pmd))) {
> +	if (unlikely(pmd_none(pmdp_get(vmf->pmd)))) {
>   		/*
>   		 * Leave __pte_alloc() until later: because vm_ops->fault may
>   		 * want to allocate huge page, and if we expose page table
> @@ -6309,13 +6309,13 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>   	if (!vmf.pud)
>   		return VM_FAULT_OOM;
>   retry_pud:
> -	if (pud_none(*vmf.pud) &&
> +	if (pud_none(pudp_get(vmf.pud)) &&
>   	    thp_vma_allowable_order(vma, vm_flags, TVA_PAGEFAULT, PUD_ORDER)) {
>   		ret = create_huge_pud(&vmf);
>   		if (!(ret & VM_FAULT_FALLBACK))
>   			return ret;
>   	} else {
> -		pud_t orig_pud = *vmf.pud;
> +		pud_t orig_pud = pudp_get(vmf.pud);
>   
>   		barrier();
>   		if (pud_trans_huge(orig_pud)) {
> @@ -6343,7 +6343,7 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>   	if (pud_trans_unstable(vmf.pud))
>   		goto retry_pud;
>   
> -	if (pmd_none(*vmf.pmd) &&
> +	if (pmd_none(pmdp_get(vmf.pmd)) &&
>   	    thp_vma_allowable_order(vma, vm_flags, TVA_PAGEFAULT, PMD_ORDER)) {
>   		ret = create_huge_pmd(&vmf);
>   		if (ret & VM_FAULT_FALLBACK)