Re: [RFC PATCH 13/26] hugetlb: add huge_pte_alloc_high_granularity

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

On 24/06/22 11:06 pm, James Houghton wrote:
> This function is to be used to do a HugeTLB page table walk where we may
> need to split a leaf-level huge PTE into a new page table level.
>
> Consider the case where we want to install 4K inside an empty 1G page:
> 1. We walk to the PUD and notice that it is pte_none.
> 2. We split the PUD by calling `hugetlb_split_to_shift`, creating a
>     standard PUD that points to PMDs that are all pte_none.
> 3. We continue the PT walk to find the PMD. We split it just like we
>     split the PUD.
> 4. We find the PTE and give it back to the caller.
>
> To avoid concurrent splitting operations on the same page table entry,
> we require that the mapping rwsem is held for writing while collapsing
> and for reading when doing a high-granularity PT walk.
>
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
>   include/linux/hugetlb.h | 23 ++++++++++++++
>   mm/hugetlb.c            | 67 +++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 90 insertions(+)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 605aa19d8572..321f5745d87f 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1176,14 +1176,37 @@ static inline void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
>   }
>   #endif	/* CONFIG_HUGETLB_PAGE */
>   
> +enum split_mode {
> +	HUGETLB_SPLIT_NEVER   = 0,
> +	HUGETLB_SPLIT_NONE    = 1 << 0,
> +	HUGETLB_SPLIT_PRESENT = 1 << 1,
huge
> +	HUGETLB_SPLIT_ALWAYS  = HUGETLB_SPLIT_NONE | HUGETLB_SPLIT_PRESENT,
> +};
>   #ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
>   /* If HugeTLB high-granularity mappings are enabled for this VMA. */
>   bool hugetlb_hgm_enabled(struct vm_area_struct *vma);
> +int huge_pte_alloc_high_granularity(struct hugetlb_pte *hpte,
> +				    struct mm_struct *mm,
> +				    struct vm_area_struct *vma,
> +				    unsigned long addr,
> +				    unsigned int desired_sz,
> +				    enum split_mode mode,
> +				    bool write_locked);
>   #else
>   static inline bool hugetlb_hgm_enabled(struct vm_area_struct *vma)
>   {
>   	return false;
>   }
> +static inline int huge_pte_alloc_high_granularity(struct hugetlb_pte *hpte,
> +					   struct mm_struct *mm,
> +					   struct vm_area_struct *vma,
> +					   unsigned long addr,
> +					   unsigned int desired_sz,
> +					   enum split_mode mode,
> +					   bool write_locked)
> +{
> +	return -EINVAL;
> +}
>   #endif
>   
>   static inline spinlock_t *huge_pte_lock(struct hstate *h,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index eaffe7b4f67c..6e0c5fbfe32c 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -7166,6 +7166,73 @@ static int hugetlb_split_to_shift(struct mm_struct *mm, struct vm_area_struct *v
>   	tlb_finish_mmu(&tlb);
>   	return ret;
>   }
> +
> +/*
> + * Similar to huge_pte_alloc except that this can be used to create or walk
> + * high-granularity mappings. It will automatically split existing HugeTLB PTEs
> + * if required by @mode. The resulting HugeTLB PTE will be returned in @hpte.
> + *
> + * There are three options for @mode:
> + *  - HUGETLB_SPLIT_NEVER   - Never split.
> + *  - HUGETLB_SPLIT_NONE    - Split empty PTEs.
> + *  - HUGETLB_SPLIT_PRESENT - Split present PTEs.
> + *  - HUGETLB_SPLIT_ALWAYS  - Split both empty and present PTEs.
> + */
> +int huge_pte_alloc_high_granularity(struct hugetlb_pte *hpte,
> +				    struct mm_struct *mm,
> +				    struct vm_area_struct *vma,
> +				    unsigned long addr,
> +				    unsigned int desired_shift,
> +				    enum split_mode mode,
> +				    bool write_locked)
> +{
> +	struct address_space *mapping = vma->vm_file->f_mapping;
> +	bool has_write_lock = write_locked;
> +	unsigned long desired_sz = 1UL << desired_shift;
> +	int ret;
> +
> +	BUG_ON(!hpte);
> +
> +	if (has_write_lock)
> +		i_mmap_assert_write_locked(mapping);
> +	else
> +		i_mmap_assert_locked(mapping);

> +
> +retry:
> +	ret = 0;
> +	hugetlb_pte_init(hpte);
> +
> +	ret = hugetlb_walk_to(mm, hpte, addr, desired_sz,
> +			      !(mode & HUGETLB_SPLIT_NONE));

hugetlb_walk_to when called with split_non mode can change mappings?

If so should be ensure we are holding write-lock here.

> +	if (ret || hugetlb_pte_size(hpte) == desired_sz)
> +		goto out;
> +
> +	if (
> +		((mode & HUGETLB_SPLIT_NONE) && hugetlb_pte_none(hpte)) ||
> +		((mode & HUGETLB_SPLIT_PRESENT) &&
> +		  hugetlb_pte_present_leaf(hpte))
> +	   ) {
> +		if (!has_write_lock) {
> +			i_mmap_unlock_read(mapping);
Should lock upgrade be used here?
> +			i_mmap_lock_write(mapping);
> +			has_write_lock = true;
> +			goto retry;
> +		}
> +		ret = hugetlb_split_to_shift(mm, vma, hpte, addr,
> +					     desired_shift);
> +	}
> +
> +out:
> +	if (has_write_lock && !write_locked) {
> +		/* Drop the write lock. */
> +		i_mmap_unlock_write(mapping);
> +		i_mmap_lock_read(mapping);
same here lock downgrade?
> +		has_write_lock = false;
> +		goto retry;
> +	}
> +
> +	return ret;
> +}
>   #endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
>   
>   /*