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SubjectRe: [PATCH v2 3/9] mm/hugetlb: make pud_huge() and huge_pud() aware of non-present pud entry
On 06/24/22 08:51, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>
> follow_pud_mask() does not support non-present pud entry now. As long as
> I tested on x86_64 server, follow_pud_mask() still simply returns
> no_page_table() for non-present_pud_entry() due to pud_bad(), so no severe
> user-visible effect should happen. But generally we should call
> follow_huge_pud() for non-present pud entry for 1GB hugetlb page.
>
> Update pud_huge() and huge_pud() to handle non-present pud entries. The
> changes are similar to previous works for pud entries commit e66f17ff7177
> ("mm/hugetlb: take page table lock in follow_huge_pmd()") and commit
> cbef8478bee5 ("mm/hugetlb: pmd_huge() returns true for non-present hugepage").
>
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> ---
> arch/x86/mm/hugetlbpage.c | 3 ++-
> mm/hugetlb.c | 26 +++++++++++++++++++++++++-
> 2 files changed, 27 insertions(+), 2 deletions(-)

Thanks. Overall looks good with typos corrected that you already noticed.
One question below.
>
> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> index a0d023cb4292..5fb86fb49ba8 100644
> --- a/arch/x86/mm/hugetlbpage.c
> +++ b/arch/x86/mm/hugetlbpage.c
> @@ -70,7 +70,8 @@ int pmd_huge(pmd_t pmd)
>
> int pud_huge(pud_t pud)
> {
> - return !!(pud_val(pud) & _PAGE_PSE);
> + return !pud_none(pud) &&
> + (pud_val(pud) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;
> }
> #endif
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f59f43c06601..b7ae5f73f3b2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6946,10 +6946,34 @@ struct page * __weak
> follow_huge_pud(struct mm_struct *mm, unsigned long address,
> pud_t *pud, int flags)
> {
> + struct page *page = NULL;
> + spinlock_t *ptl;
> + pte_t pte;
> +
> if (flags & (FOLL_GET | FOLL_PIN))
> return NULL;
>
> - return pte_page(*(pte_t *)pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
> +retry:
> + ptl = huge_pte_lock(hstate_sizelog(PUD_SHIFT), mm, (pte_t *)pud);
> + if (!pud_huge(*pud))
> + goto out;
> + pte = huge_ptep_get((pte_t *)pud);
> + if (pte_present(pte)) {
> + page = pud_page(*pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
> + if (WARN_ON_ONCE(!try_grab_page(page, flags))) {

The call to try_grab_page() seems a bit strange since flags will not include
FOLL_GET | FOLL_PIN as tested above. Is try_grab_page() always going be a
noop here?

--
Mike Kravetz

> + page = NULL;
> + goto out;
> + }
> + } else {
> + if (is_hugetlb_entry_migration(pte)) {
> + spin_unlock(ptl);
> + __migration_entry_wait(mm, (pte_t *)pud, ptl);
> + goto retry;
> + }
> + }
> +out:
> + spin_unlock(ptl);
> + return page;
> }
>
> struct page * __weak
> --
> 2.25.1
>

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