Re: [RFC PATCH v4 7/8] hugetlb: create hugetlb_unmap_file_folio to unmap single file folio

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/7/7 4:23, 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 | 124 +++++++++++++++++++++++++++++++++----------
>  1 file changed, 95 insertions(+), 29 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 31bd4325fce5..0eac0ea2a245 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -396,6 +396,94 @@ static int hugetlbfs_write_end(struct file *file, struct address_space *mapping,
>  	return -EINVAL;
>  }
>  
> +/*
> + * 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;
> +
> +	return false;	/* WTH??? */

I'm sorry but what does WTH means? IIUC, this could happen if pte_page is a COW-ed private page?
vma_interval_tree_foreach doesn't exclude the private mapping even after cow?

Except from above (trivial one), this patch looks good to me.

Thanks.