Re: [PATCH v2] mm/hugetlb: fix hugetlb vs. core-mm PT locking

[James Houghton]

On Tue, Jul 30, 2024 at 2:07 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 30.07.24 23:00, David Hildenbrand wrote:
> > On 30.07.24 22:43, James Houghton wrote:
> >> On Tue, Jul 30, 2024 at 1:03 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >>> index b100df8cb5857..1b1f40ff00b7d 100644
> >>> --- a/include/linux/mm.h
> >>> +++ b/include/linux/mm.h
> >>> @@ -2926,6 +2926,12 @@ static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
> >>>           return ptlock_ptr(page_ptdesc(pmd_page(*pmd)));
> >>>    }
> >>>
> >>> +static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
> >>> +{
> >>> +       BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHPTE));
> >>> +       return ptlock_ptr(virt_to_ptdesc(pte));
> >>
> >> Hi David,
> >>
> >
> > Hi!
> >
> >> Small question: ptep_lockptr() does not handle the case where the size
> >> of the PTE table is larger than PAGE_SIZE, but pmd_lockptr() does.
> >
> > I thought I convinced myself that leaf page tables are always single
> > pages and had a comment in v1.
> >
> > But now I have to double-check again, and staring at
> > pagetable_pte_ctor() callers I am left confused.
> >
> > It certainly sounds more future proof to just align the pointer down to
> > the start of the PTE table like pmd_lockptr() would.
> >
> >> IIUC, for pte_lockptr() and ptep_lockptr() to return the same result
> >> in this case, ptep_lockptr() should be doing the masking that
> >> pmd_lockptr() is doing. Are you sure that you don't need to be doing
> >> it? (Or maybe I am misunderstanding something.)
> >
> > It's a valid concern even if it would not be required. But I'm afraid I
> > won't dig into the details and simply do the alignment in a v3.
>
> To be precise, the following on top:
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1b1f40ff00b7d..f6c7fe8f5746f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2926,10 +2926,22 @@ static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
>          return ptlock_ptr(page_ptdesc(pmd_page(*pmd)));
>   }
>
> -static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
> +static inline struct page *ptep_pgtable_page(pte_t *pte)
>   {
> +       unsigned long mask = ~(PTRS_PER_PTE * sizeof(pte_t) - 1);
> +
>          BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHPTE));
> -       return ptlock_ptr(virt_to_ptdesc(pte));
> +       return virt_to_page((void *)((unsigned long)pte & mask));
> +}
> +
> +static inline struct ptdesc *ptep_ptdesc(pte_t *pte)
> +{
> +       return page_ptdesc(ptep_pgtable_page(pte));
> +}
> +
> +static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
> +{
> +       return ptlock_ptr(ptep_ptdesc(pte));
>   }

Thanks! That looks right to me. Feel free to add

Reviewed-by: James Houghton <jthoughton@xxxxxxxxxx>

>
>
> virt_to_ptdesc() really is of limited use in core-mm code as it seems ...
>
> --
> Cheers,
>
> David / dhildenb
>