Re: [PATCH 7/8] hugetlb: create hugetlb_unmap_file_folio to unmap single file folio

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/8/25 1:57, Mike Kravetz wrote:
> Create the new routine hugetlb_unmap_file_folio that will unmap a single
> file folio.  This is refactored code from hugetlb_vmdelete_list.  It is
> modified to do locking within the routine itself and check whether the
> page is mapped within a specific vma before unmapping.
> 
> This refactoring will be put to use and expanded upon in a subsequent
> patch adding vma specific locking.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  fs/hugetlbfs/inode.c | 123 +++++++++++++++++++++++++++++++++----------
>  1 file changed, 94 insertions(+), 29 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index e83fd31671b3..b93d131b0cb5 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -371,6 +371,94 @@ static void hugetlb_delete_from_page_cache(struct page *page)
>  	delete_from_page_cache(page);
>  }
>  
> +/*
> + * Called with i_mmap_rwsem held for inode based vma maps.  This makes
> + * sure vma (and vm_mm) will not go away.  We also hold the hugetlb fault
> + * 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)
> +{
> +	pte_t *ptep, pte;
> +
> +	ptep = huge_pte_offset(vma->vm_mm, addr,
> +			huge_page_size(hstate_vma(vma)));
> +
> +	if (!ptep)
> +		return false;
> +
> +	pte = huge_ptep_get(ptep);
> +	if (huge_pte_none(pte) || !pte_present(pte))
> +		return false;
> +
> +	if (pte_page(pte) == page)
> +		return true;

I'm thinking whether pte entry could change after we check it since huge_pte_lock is not held here.
But I think holding i_mmap_rwsem in writelock mode should give us such a guarantee, e.g. migration
entry is changed back to huge pte entry while holding i_mmap_rwsem in readlock mode.
Or am I miss something?

> +
> +	return false;
> +}
> +
> +/*
> + * Can vma_offset_start/vma_offset_end overflow on 32-bit arches?
> + * No, because the interval tree returns us only those vmas
> + * which overlap the truncated area starting at pgoff,
> + * and no vma on a 32-bit arch can span beyond the 4GB.
> + */
> +static unsigned long vma_offset_start(struct vm_area_struct *vma, pgoff_t start)
> +{
> +	if (vma->vm_pgoff < start)
> +		return (start - vma->vm_pgoff) << PAGE_SHIFT;
> +	else
> +		return 0;
> +}
> +
> +static unsigned long vma_offset_end(struct vm_area_struct *vma, pgoff_t end)
> +{
> +	unsigned long t_end;
> +
> +	if (!end)
> +		return vma->vm_end;
> +
> +	t_end = ((end - vma->vm_pgoff) << PAGE_SHIFT) + vma->vm_start;
> +	if (t_end > vma->vm_end)
> +		t_end = vma->vm_end;
> +	return t_end;
> +}
> +
> +/*
> + * Called with hugetlb fault mutex held.  Therefore, no more mappings to
> + * this folio can be created while executing the routine.
> + */
> +static void hugetlb_unmap_file_folio(struct hstate *h,
> +					struct address_space *mapping,
> +					struct folio *folio, pgoff_t index)
> +{
> +	struct rb_root_cached *root = &mapping->i_mmap;
> +	struct page *page = &folio->page;
> +	struct vm_area_struct *vma;
> +	unsigned long v_start;
> +	unsigned long v_end;
> +	pgoff_t start, end;
> +
> +	start = index * pages_per_huge_page(h);
> +	end = ((index + 1) * pages_per_huge_page(h));

It seems the outer parentheses is unneeded?

Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>

Thanks,
Miaohe Lin