linux-kernel - Re: [RFC PATCH v4 7/8] hugetlb: create hugetlb_unmap_file

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Date:   Fri, 29 Jul 2022 10:02:48 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
CC:     Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Hildenbrand <david@...hat.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Ray Fucillo <Ray.Fucillo@...ersystems.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH v4 7/8] hugetlb: create hugetlb_unmap_file_folio to
 unmap single file folio

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@...cle.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.