Re: [RFC PATCH v2 15/47] hugetlbfs: for unmapping, treat HGM-mapped pages as potentially mapped

[Mike Kravetz]

On 10/21/22 16:36, James Houghton wrote:
> hugetlb_vma_maps_page was mostly used as an optimization: if the VMA
> isn't mapping a page, then we don't have to attempt to unmap it again.
> We are still able to call the unmap routine if we need to.
> 
> For high-granularity mapped pages, we can't easily do a full walk to see
> if the page is actually mapped or not, so simply return that it might
> be.
> 
> Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> ---
>  fs/hugetlbfs/inode.c | 27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 7f836f8f9db1..a7ab62e39b8c 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -383,21 +383,34 @@ static void hugetlb_delete_from_page_cache(struct folio *folio)
>   * mutex for the page in the mapping.  So, we can not race with page being
>   * faulted into the vma.
>   */
> -static bool hugetlb_vma_maps_page(struct vm_area_struct *vma,
> -				unsigned long addr, struct page *page)
> +static bool hugetlb_vma_maybe_maps_page(struct vm_area_struct *vma,
> +					unsigned long addr, struct page *page)
>  {
>  	pte_t *ptep, pte;
> +	struct hugetlb_pte hpte;
> +	struct hstate *h = hstate_vma(vma);
>  
> -	ptep = huge_pte_offset(vma->vm_mm, addr,
> -			huge_page_size(hstate_vma(vma)));
> +	ptep = huge_pte_offset(vma->vm_mm, addr, huge_page_size(h));
>  
>  	if (!ptep)
>  		return false;
>  
> +	hugetlb_pte_populate(&hpte, ptep, huge_page_shift(h),
> +			hpage_size_to_level(huge_page_size(h)));
> +

Only a nit, the hugetlb_pte_populate call should probably be after the
check below.

This makes sense, and as you mention hugetlb_vma_maybe_maps_page is mostly an
optimization that will still work as designed for non HGM vmas.

-- 
Mike Kravetz

>  	pte = huge_ptep_get(ptep);
>  	if (huge_pte_none(pte) || !pte_present(pte))
>  		return false;
>  
> +	if (!hugetlb_pte_present_leaf(&hpte, pte))
> +		/*
> +		 * The top-level PTE is not a leaf, so it's possible that a PTE
> +		 * under us is mapping the page. We aren't holding the VMA
> +		 * lock, so it is unsafe to continue the walk further. Instead,
> +		 * return true to indicate that we might be mapping the page.
> +		 */
> +		return true;
> +
>  	if (pte_page(pte) == page)
>  		return true;
>  
> @@ -457,7 +470,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
>  		v_start = vma_offset_start(vma, start);
>  		v_end = vma_offset_end(vma, end);
>  
> -		if (!hugetlb_vma_maps_page(vma, vma->vm_start + v_start, page))
> +		if (!hugetlb_vma_maybe_maps_page(vma, vma->vm_start + v_start,
> +					page))
>  			continue;
>  
>  		if (!hugetlb_vma_trylock_write(vma)) {
> @@ -507,7 +521,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
>  		 */
>  		v_start = vma_offset_start(vma, start);
>  		v_end = vma_offset_end(vma, end);
> -		if (hugetlb_vma_maps_page(vma, vma->vm_start + v_start, page))
> +		if (hugetlb_vma_maybe_maps_page(vma, vma->vm_start + v_start,
> +					page))
>  			unmap_hugepage_range(vma, vma->vm_start + v_start,
>  						v_end, NULL,
>  						ZAP_FLAG_DROP_MARKER);
> -- 
> 2.38.0.135.g90850a2211-goog
>