[PATCH v5 2/2] mm: Allocate THP on hugezeropage wp-fault

[Kefeng Wang]

On 2024/9/24 18:16, Dev Jain wrote:
> Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
> replace it with a PMD-mapped THP. Remember to flush TLB entry
> corresponding to the hugezeropage. In case of failure, fallback
> to splitting the PMD.
> 
> Signed-off-by: Dev Jain <dev.jain at arm.com>
> ---
>   mm/huge_memory.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bdbf67c18f6c..fbb195bc2038 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1790,6 +1790,41 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
>   	spin_unlock(vmf->ptl);
>   }
>   
> +static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf)
> +{
> +	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> +	struct vm_area_struct *vma = vmf->vma;
> +	struct mmu_notifier_range range;
> +	struct folio *folio;
> +	vm_fault_t ret = 0;
> +
> +	folio = vma_alloc_anon_folio_pmd(vma, vmf->address);
> +	if (unlikely(!folio)) {
> +		ret = VM_FAULT_FALLBACK;

I'd like to return VM_FAULT_FALLBACK if you re-post.

Reviewed-by: Kefeng Wang <wangkefeng.wang at huawei.com>

> +		goto out;
> +	}
> +
> +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
> +				haddr + HPAGE_PMD_SIZE);
> +	mmu_notifier_invalidate_range_start(&range);
> +	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> +	if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
> +		goto release;
> +	ret = check_stable_address_space(vma->vm_mm);
> +	if (ret)
> +		goto release;
> +	(void)pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
> +	map_anon_folio_pmd(folio, vmf->pmd, vma, haddr);
> +	goto unlock;
> +release:
> +	folio_put(folio);
> +unlock:
> +	spin_unlock(vmf->ptl);
> +	mmu_notifier_invalidate_range_end(&range);
> +out:
> +	return ret;
> +}
> +
>   vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>   {
>   	const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
> @@ -1802,8 +1837,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>   	vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
>   	VM_BUG_ON_VMA(!vma->anon_vma, vma);
>   
> -	if (is_huge_zero_pmd(orig_pmd))
> +	if (is_huge_zero_pmd(orig_pmd)) {
> +		vm_fault_t ret = do_huge_zero_wp_pmd(vmf);
> +
> +		if (!(ret & VM_FAULT_FALLBACK))
> +			return ret;
> +
> +		/* Fallback to splitting PMD if THP cannot be allocated */
>   		goto fallback;
> +	}
>   
>   	spin_lock(vmf->ptl);
>