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SubjectRe: [RFC PATCH v2 15/47] hugetlbfs: for unmapping, treat HGM-mapped pages as potentially mapped
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@google.com>
> ---
> 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
>

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