Re: [RFC PATCH 04/26] hugetlb: make huge_pte_lockptr take an explicit shift argument.

["manish.mishra" <manish.mishra@nutanix.com>]

On 24/06/22 11:06 pm, James Houghton wrote:
> This is needed to handle PTL locking with high-granularity mapping. We
> won't always be using the PMD-level PTL even if we're using the 2M
> hugepage hstate. It's possible that we're dealing with 4K PTEs, in which
> case, we need to lock the PTL for the 4K PTE.
>
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
>   arch/powerpc/mm/pgtable.c |  3 ++-
>   include/linux/hugetlb.h   | 19 ++++++++++++++-----
>   mm/hugetlb.c              |  9 +++++----
>   mm/migrate.c              |  3 ++-
>   mm/page_vma_mapped.c      |  3 ++-
>   5 files changed, 25 insertions(+), 12 deletions(-)
>
> diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
> index e6166b71d36d..663d591a8f08 100644
> --- a/arch/powerpc/mm/pgtable.c
> +++ b/arch/powerpc/mm/pgtable.c
> @@ -261,7 +261,8 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>   
>   		psize = hstate_get_psize(h);
>   #ifdef CONFIG_DEBUG_VM
> -		assert_spin_locked(huge_pte_lockptr(h, vma->vm_mm, ptep));
> +		assert_spin_locked(huge_pte_lockptr(huge_page_shift(h),
> +						    vma->vm_mm, ptep));
>   #endif
>   
>   #else
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 498a4ae3d462..5fe1db46d8c9 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -868,12 +868,11 @@ static inline gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t gfp_mask)
>   	return modified_mask;
>   }
>   
> -static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
> +static inline spinlock_t *huge_pte_lockptr(unsigned int shift,
>   					   struct mm_struct *mm, pte_t *pte)
>   {
> -	if (huge_page_size(h) == PMD_SIZE)
> +	if (shift == PMD_SHIFT)
>   		return pmd_lockptr(mm, (pmd_t *) pte);
> -	VM_BUG_ON(huge_page_size(h) == PAGE_SIZE);

I may have wrong understanding here, is per pmd lock for reducing

contention, if that is the case should be take per pmd lock of

PAGE_SIZE too and page_table_lock for anything higer than PMD.

>   	return &mm->page_table_lock;
>   }
>   
> @@ -1076,7 +1075,7 @@ static inline gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t gfp_mask)
>   	return 0;
>   }
>   
> -static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
> +static inline spinlock_t *huge_pte_lockptr(unsigned int shift,
>   					   struct mm_struct *mm, pte_t *pte)
>   {
>   	return &mm->page_table_lock;
> @@ -1116,7 +1115,17 @@ static inline spinlock_t *huge_pte_lock(struct hstate *h,
>   {
>   	spinlock_t *ptl;
>   
> -	ptl = huge_pte_lockptr(h, mm, pte);
> +	ptl = huge_pte_lockptr(huge_page_shift(h), mm, pte);
> +	spin_lock(ptl);
> +	return ptl;
> +}
> +
> +static inline spinlock_t *huge_pte_lock_shift(unsigned int shift,
> +					      struct mm_struct *mm, pte_t *pte)
> +{
> +	spinlock_t *ptl;
> +
> +	ptl = huge_pte_lockptr(shift, mm, pte);
>   	spin_lock(ptl);
>   	return ptl;
>   }
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 0eec34edf3b2..d6d0d4c03def 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4817,7 +4817,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>   			continue;
>   
>   		dst_ptl = huge_pte_lock(h, dst, dst_pte);
> -		src_ptl = huge_pte_lockptr(h, src, src_pte);
> +		src_ptl = huge_pte_lockptr(huge_page_shift(h), src, src_pte);
>   		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
>   		entry = huge_ptep_get(src_pte);
>   		dst_entry = huge_ptep_get(dst_pte);
> @@ -4894,7 +4894,8 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>   
>   				/* Install the new huge page if src pte stable */
>   				dst_ptl = huge_pte_lock(h, dst, dst_pte);
> -				src_ptl = huge_pte_lockptr(h, src, src_pte);
> +				src_ptl = huge_pte_lockptr(huge_page_shift(h),
> +							   src, src_pte);
>   				spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
>   				entry = huge_ptep_get(src_pte);
>   				if (!pte_same(src_pte_old, entry)) {
> @@ -4948,7 +4949,7 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr,
>   	pte_t pte;
>   
>   	dst_ptl = huge_pte_lock(h, mm, dst_pte);
> -	src_ptl = huge_pte_lockptr(h, mm, src_pte);
> +	src_ptl = huge_pte_lockptr(huge_page_shift(h), mm, src_pte);
>   
>   	/*
>   	 * We don't have to worry about the ordering of src and dst ptlocks
> @@ -6024,7 +6025,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>   		page_in_pagecache = true;
>   	}
>   
> -	ptl = huge_pte_lockptr(h, dst_mm, dst_pte);
> +	ptl = huge_pte_lockptr(huge_page_shift(h), dst_mm, dst_pte);
>   	spin_lock(ptl);
>   
>   	/*
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e51588e95f57..a8a960992373 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -318,7 +318,8 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
>   void migration_entry_wait_huge(struct vm_area_struct *vma,
>   		struct mm_struct *mm, pte_t *pte)
>   {
> -	spinlock_t *ptl = huge_pte_lockptr(hstate_vma(vma), mm, pte);
> +	spinlock_t *ptl = huge_pte_lockptr(huge_page_shift(hstate_vma(vma)),
> +					   mm, pte);
>   	__migration_entry_wait(mm, pte, ptl);
>   }
>   
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index c10f839fc410..8921dd4e41b1 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -174,7 +174,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>   		if (!pvmw->pte)
>   			return false;
>   
> -		pvmw->ptl = huge_pte_lockptr(hstate, mm, pvmw->pte);
> +		pvmw->ptl = huge_pte_lockptr(huge_page_shift(hstate),
> +					     mm, pvmw->pte);
>   		spin_lock(pvmw->ptl);
>   		if (!check_pte(pvmw))
>   			return not_found(pvmw);