Re: [RFC PATCH 17/26] hugetlb: update follow_hugetlb_page to support HGM

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

On 24/06/22 11:06 pm, James Houghton wrote:
> This enables support for GUP, and it is needed for the KVM demand paging
> self-test to work.
>
> One important change here is that, before, we never needed to grab the
> i_mmap_sem, but now, to prevent someone from collapsing the page tables
> out from under us, we grab it for reading when doing high-granularity PT
> walks.
>
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
>   mm/hugetlb.c | 70 ++++++++++++++++++++++++++++++++++++++++++----------
>   1 file changed, 57 insertions(+), 13 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f9c7daa6c090..aadfcee947cf 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6298,14 +6298,18 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   	unsigned long vaddr = *position;
>   	unsigned long remainder = *nr_pages;
>   	struct hstate *h = hstate_vma(vma);
> +	struct address_space *mapping = vma->vm_file->f_mapping;
>   	int err = -EFAULT, refs;
> +	bool has_i_mmap_sem = false;
>   
>   	while (vaddr < vma->vm_end && remainder) {
>   		pte_t *pte;
>   		spinlock_t *ptl = NULL;
>   		bool unshare = false;
>   		int absent;
> +		unsigned long pages_per_hpte;
>   		struct page *page;
> +		struct hugetlb_pte hpte;
>   
>   		/*
>   		 * If we have a pending SIGKILL, don't keep faulting pages and
> @@ -6325,9 +6329,23 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   		 */
>   		pte = huge_pte_offset(mm, vaddr & huge_page_mask(h),
>   				      huge_page_size(h));
> -		if (pte)
> -			ptl = huge_pte_lock(h, mm, pte);
> -		absent = !pte || huge_pte_none(huge_ptep_get(pte));
> +		if (pte) {
> +			hugetlb_pte_populate(&hpte, pte, huge_page_shift(h));
> +			if (hugetlb_hgm_enabled(vma)) {
> +				BUG_ON(has_i_mmap_sem);

Just thinking can we do without i_mmap_lock_read in most cases. Like earlier

this function was good without i_mmap_lock_read doing almost everything

which is happening now?

> +				i_mmap_lock_read(mapping);
> +				/*
> +				 * Need to hold the mapping semaphore for
> +				 * reading to do a HGM walk.
> +				 */
> +				has_i_mmap_sem = true;
> +				hugetlb_walk_to(mm, &hpte, vaddr, PAGE_SIZE,
> +						/*stop_at_none=*/true);
> +			}
> +			ptl = hugetlb_pte_lock(mm, &hpte);
> +		}
> +
> +		absent = !pte || hugetlb_pte_none(&hpte);
>   
>   		/*
>   		 * When coredumping, it suits get_dump_page if we just return
> @@ -6338,8 +6356,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   		 */
>   		if (absent && (flags & FOLL_DUMP) &&
>   		    !hugetlbfs_pagecache_present(h, vma, vaddr)) {
> -			if (pte)
> +			if (pte) {
> +				if (has_i_mmap_sem) {
> +					i_mmap_unlock_read(mapping);
> +					has_i_mmap_sem = false;
> +				}
>   				spin_unlock(ptl);
> +			}
>   			remainder = 0;
>   			break;
>   		}
> @@ -6359,8 +6382,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   			vm_fault_t ret;
>   			unsigned int fault_flags = 0;
>   
> -			if (pte)
> +			if (pte) {
> +				if (has_i_mmap_sem) {
> +					i_mmap_unlock_read(mapping);
> +					has_i_mmap_sem = false;
> +				}
>   				spin_unlock(ptl);
> +			}
>   			if (flags & FOLL_WRITE)
>   				fault_flags |= FAULT_FLAG_WRITE;
>   			else if (unshare)
> @@ -6403,8 +6431,11 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   			continue;
>   		}
>   
> -		pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
> -		page = pte_page(huge_ptep_get(pte));
> +		pfn_offset = (vaddr & ~hugetlb_pte_mask(&hpte)) >> PAGE_SHIFT;
> +		page = pte_page(hugetlb_ptep_get(&hpte));
> +		pages_per_hpte = hugetlb_pte_size(&hpte) / PAGE_SIZE;
> +		if (hugetlb_hgm_enabled(vma))
> +			page = compound_head(page);
>   
>   		VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
>   			       !PageAnonExclusive(page), page);
> @@ -6414,17 +6445,21 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   		 * and skip the same_page loop below.
>   		 */
>   		if (!pages && !vmas && !pfn_offset &&
> -		    (vaddr + huge_page_size(h) < vma->vm_end) &&
> -		    (remainder >= pages_per_huge_page(h))) {
> -			vaddr += huge_page_size(h);
> -			remainder -= pages_per_huge_page(h);
> -			i += pages_per_huge_page(h);
> +		    (vaddr + pages_per_hpte < vma->vm_end) &&
> +		    (remainder >= pages_per_hpte)) {
> +			vaddr += pages_per_hpte;
> +			remainder -= pages_per_hpte;
> +			i += pages_per_hpte;
>   			spin_unlock(ptl);
> +			if (has_i_mmap_sem) {
> +				has_i_mmap_sem = false;
> +				i_mmap_unlock_read(mapping);
> +			}
>   			continue;
>   		}
>   
>   		/* vaddr may not be aligned to PAGE_SIZE */
> -		refs = min3(pages_per_huge_page(h) - pfn_offset, remainder,
> +		refs = min3(pages_per_hpte - pfn_offset, remainder,
>   		    (vma->vm_end - ALIGN_DOWN(vaddr, PAGE_SIZE)) >> PAGE_SHIFT);
>   
>   		if (pages || vmas)
> @@ -6447,6 +6482,10 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   			if (WARN_ON_ONCE(!try_grab_folio(pages[i], refs,
>   							 flags))) {
>   				spin_unlock(ptl);
> +				if (has_i_mmap_sem) {
> +					has_i_mmap_sem = false;
> +					i_mmap_unlock_read(mapping);
> +				}
>   				remainder = 0;
>   				err = -ENOMEM;
>   				break;
> @@ -6458,8 +6497,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   		i += refs;
>   
>   		spin_unlock(ptl);
> +		if (has_i_mmap_sem) {
> +			has_i_mmap_sem = false;
> +			i_mmap_unlock_read(mapping);
> +		}
>   	}
>   	*nr_pages = remainder;
> +	BUG_ON(has_i_mmap_sem);
>   	/*
>   	 * setting position is actually required only if remainder is
>   	 * not zero but it's faster not to add a "if (remainder)"

Thanks

Manish Mishra