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

[David Hildenbrand]

On 30.07.24 23:17, James Houghton wrote:
> 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>

Thanks for the review, will send a v3 tomorrow after having wasted more 
valuable life time setting up the ARM environment again ... :)

--
Cheers,

David / dhildenb