Re: [PATCH 04/10] mm/hugetlb: Move swap entry handling into vma lock when faulted

Peter Xu
2022-12-05T23:38:21Z

On Mon, Dec 05, 2022 at 02:14:38PM -0800, Mike Kravetz wrote:
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index dfe677fadaf8..776e34ccf029 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -5826,22 +5826,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > int need_wait_lock = 0;
> > unsigned long haddr = address & huge_page_mask(h);
> >
> > - ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
> > - if (ptep) {
> > - /*
> > - * Since we hold no locks, ptep could be stale. That is
> > - * OK as we are only making decisions based on content and
> > - * not actually modifying content here.
> > - */
> > - entry = huge_ptep_get(ptep);
> > - if (unlikely(is_hugetlb_entry_migration(entry))) {
> > - migration_entry_wait_huge(vma, ptep);
> > - return 0;
> > - } else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
> > - return VM_FAULT_HWPOISON_LARGE |
> > - VM_FAULT_SET_HINDEX(hstate_index(h));
> > - }
> > -
>
> Before acquiring the vma_lock, there is this comment:
>
> /*
> * Acquire vma lock before calling huge_pte_alloc and hold
> * until finished with ptep. This prevents huge_pmd_unshare from
> * being called elsewhere and making the ptep no longer valid.
> *
> * ptep could have already be assigned via hugetlb_walk(). That
> * is OK, as huge_pte_alloc will return the same value unless
> * something has changed.
> */
>
> The second sentence in that comment about ptep being already assigned no
> longer applies and can be deleted.

Correct, this can be removed.

One thing to mention is there's an inline touch-up above in the last patch
of the series when introducing hugetlb_walk() to s/pte_offset/walk/, but I
saw that Andrew has already done the right thing on the fixup one in his
local tree, so I think we're all good.

Thanks!

--
Peter Xu