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SubjectRe: [PATCH v2 3/9] mm/hugetlb: make pud_huge() and huge_pud() aware of non-present pud entry
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On Sat, Jun 25, 2022 at 05:42:17PM +0800, Miaohe Lin wrote:
> On 2022/6/24 7:51, 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(-)
> >
> > 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)
> >
>
> No strong opinion but a comment similar to pmd_huge might be better?
>
> /*
> * pmd_huge() returns 1 if @pmd is hugetlb related entry, that is normal
> * hugetlb entry or non-present (migration or hwpoisoned) hugetlb entry.
> * Otherwise, returns 0.
> */

OK, I'll add some.

>
> > 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;
>
> Should the above check be modified? It seems the below try_grab_page might not grab the page as
> expected (as Mike pointed out). Or the extra page refcnt is unneeded?

Yes, this check should be updated.

>
> >
> > - 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))) {
> > + page = NULL;
> > + goto out;
> > + }
> > + } else {
> > + if (is_hugetlb_entry_migration(pte)) {
> > + spin_unlock(ptl);
> > + __migration_entry_wait(mm, (pte_t *)pud, ptl);
> > + goto retry;
> > + }
>
> Again. No strong opinion but a comment similar to follow_huge_pmd might be better?
>
> /*
> * hwpoisoned entry is treated as no_page_table in
> * follow_page_mask().
> */

Will add comment on this too. Thank you.

- Naoya Horiguchi
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