"
On Mon, Feb 27, 2023 at 11:17 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 02/18/23 00:27, James Houghton wrote:
> > These functions are used to allocate new PTEs below the hstate PTE. This
> > will be used by hugetlb_walk_step, which implements stepping forwards in
> > a HugeTLB high-granularity page table walk.
> >
> > The reasons that we don't use the standard pmd_alloc/pte_alloc*
> > functions are:
> > 1) This prevents us from accidentally overwriting swap entries or
> > attempting to use swap entries as present non-leaf PTEs (see
> > pmd_alloc(); we assume that !pte_none means pte_present and
> > non-leaf).
> > 2) Locking hugetlb PTEs can different than regular PTEs. (Although, as
> > implemented right now, locking is the same.)
> > 3) We can maintain compatibility with CONFIG_HIGHPTE. That is, HugeTLB
> > HGM won't use HIGHPTE, but the kernel can still be built with it,
> > and other mm code will use it.
> >
> > When GENERAL_HUGETLB supports P4D-based hugepages, we will need to
> > implement hugetlb_pud_alloc to implement hugetlb_walk_step.
> >
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index eeacadf3272b..9d839519c875 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -72,6 +72,11 @@ unsigned long hugetlb_pte_mask(const struct hugetlb_pte *hpte)
> >
> > bool hugetlb_pte_present_leaf(const struct hugetlb_pte *hpte, pte_t pte);
> >
> > +pmd_t *hugetlb_alloc_pmd(struct mm_struct *mm, struct hugetlb_pte *hpte,
> > + unsigned long addr);
> > +pte_t *hugetlb_alloc_pte(struct mm_struct *mm, struct hugetlb_pte *hpte,
> > + unsigned long addr);
> > +
> > struct hugepage_subpool {
> > spinlock_t lock;
> > long count;
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 6c74adff43b6..bb424cdf79e4 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -483,6 +483,120 @@ static bool has_same_uncharge_info(struct file_region *rg,
> > #endif
> > }
> >
> > +/*
> > + * hugetlb_alloc_pmd -- Allocate or find a PMD beneath a PUD-level hpte.
> > + *
> > + * This is meant to be used to implement hugetlb_walk_step when one must go to
> > + * step down to a PMD. Different architectures may implement hugetlb_walk_step
> > + * differently, but hugetlb_alloc_pmd and hugetlb_alloc_pte are architecture-
> > + * independent.
> > + *
> > + * Returns:
> > + * On success: the pointer to the PMD. This should be placed into a
> > + * hugetlb_pte. @hpte is not changed.
> > + * ERR_PTR(-EINVAL): hpte is not PUD-level
> > + * ERR_PTR(-EEXIST): there is a non-leaf and non-empty PUD in @hpte
>
> I often get this confused, should this really be 'non-leaf'? Because, ...
This comment is wrong, it should be "non-empty PUD that is not
pointing to page tables". Maybe it would be better to say "-EEXIST
unless @hpte is pud_none() or already points to page tables".
In this commit, PTEs containing PTE markers are treated as non-empty
here (and not pointing to page tables), but after the commit "hugetlb:
split PTE markers when doing HGM walks", they are treated as empty.
I'll update the comment in that commit as well.
>
> > + * ERR_PTR(-ENOMEM): could not allocate the new PMD
> > + */
> > +pmd_t *hugetlb_alloc_pmd(struct mm_struct *mm, struct hugetlb_pte *hpte,
> > + unsigned long addr)
> > +{
> > + spinlock_t *ptl = hugetlb_pte_lockptr(hpte);
> > + pmd_t *new;
> > + pud_t *pudp;
> > + pud_t pud;
> > +
> > + if (hpte->level != HUGETLB_LEVEL_PUD)
> > + return ERR_PTR(-EINVAL);
> > +
> > + pudp = (pud_t *)hpte->ptep;
> > +retry:
> > + pud = READ_ONCE(*pudp);
> > + if (likely(pud_present(pud)))
> > + return unlikely(pud_leaf(pud))
> > + ? ERR_PTR(-EEXIST)
> > + : pmd_offset(pudp, addr);
>
> ... it seems we return -EEXIST in the pud_leaf case.
This code is correct. :) We don't want to overwrite a leaf. Sorry for
the confusion!
