Re: [PATCH] mm: Split huge PUD on wp_huge_pud fallback

[Thomas Hellström]

On 6/23/22 07:24, Gowans, James wrote:
> Currently the implementation will split the PUD when a fallback is taken
> inside the create_huge_pud function. This isn't where it should be done:
> the splitting should be done in wp_huge_pud, just like it's done for
> PMDs. Reason being that if a callback is taken during create, there is
> no PUD yet so nothing to split, whereas if a fallback is taken when
> encountering a write protection fault there is something to split.
>
> It looks like this was the original intention with the commit where the
> splitting was introduced, but somehow it got moved to the wrong place
> between v1 and v2 of the patch series. Rebase mistake perhaps.
>
> Fixes: 327e9fd48972 ("mm: Split huge pages on write-notify or COW")

Some time since I looked into this, but looks correct to me.

Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>

> Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Christian König <christian.koenig@xxxxxxx>
> Cc: Jan H. Schönherr <jschoenh@xxxxxxxxx>
> Signed-off-by: James Gowans <jgowans@xxxxxxxxxx>
> ---
>   mm/memory.c | 31 ++++++++++++++++---------------
>   1 file changed, 16 insertions(+), 15 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 7a089145cad4..4cf7d4b6c950 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4802,29 +4802,30 @@ static vm_fault_t create_huge_pud(struct vm_fault *vmf)
>   	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
>   	/* No support for anonymous transparent PUD pages yet */
>   	if (vma_is_anonymous(vmf->vma))
> -		goto split;
> -	if (vmf->vma->vm_ops->huge_fault) {
> -		vm_fault_t ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
> -
> -		if (!(ret & VM_FAULT_FALLBACK))
> -			return ret;
> -	}
> -split:
> -	/* COW or write-notify not handled on PUD level: split pud.*/
> -	__split_huge_pud(vmf->vma, vmf->pud, vmf->address);
> +		return VM_FAULT_FALLBACK;
> +	if (vmf->vma->vm_ops->huge_fault)
> +		return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
>   #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>   	return VM_FAULT_FALLBACK;
>   }
>   
>   static vm_fault_t wp_huge_pud(struct vm_fault *vmf, pud_t orig_pud)
>   {
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) &&			\
> +	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
>   	/* No support for anonymous transparent PUD pages yet */
>   	if (vma_is_anonymous(vmf->vma))
> -		return VM_FAULT_FALLBACK;
> -	if (vmf->vma->vm_ops->huge_fault)
> -		return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
> -#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +		goto split;
> +	if (vmf->vma->vm_ops->huge_fault) {
> +		vm_fault_t ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
> +
> +		if (!(ret & VM_FAULT_FALLBACK))
> +			return ret;
> +	}
> +split:
> +	/* COW or write-notify not handled on PUD level: split pud.*/
> +	__split_huge_pud(vmf->vma, vmf->pud, vmf->address);
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE && CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
>   	return VM_FAULT_FALLBACK;
>   }
>