Re: [RFC PATCH 5/6] mm, pgtable: Add the reference counter for COW PTE

[Chih-En Lin]

On Fri, May 20, 2022 at 02:30:29PM +0000, Christophe Leroy wrote:
> 
> 
> Le 19/05/2022 à 20:31, Chih-En Lin a écrit :
> > Add the reference counter cow_pgtable_refcount to maintain the number
> > of process references to COW PTE. Before decreasing the reference
> > count, it will check whether the counter is one or not for reusing
> > COW PTE when the counter is one.
> > 
> > Signed-off-by: Chih-En Lin <shiyn.lin@xxxxxxxxx>
> > ---
> >   include/linux/mm.h       |  1 +
> >   include/linux/mm_types.h |  1 +
> >   include/linux/pgtable.h  | 27 +++++++++++++++++++++++++++
> >   mm/memory.c              |  1 +
> >   4 files changed, 30 insertions(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 221926a3d818..e48bb3fbc33c 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2329,6 +2329,7 @@ static inline bool pgtable_pte_page_ctor(struct page *page)
> >          __SetPageTable(page);
> >          inc_lruvec_page_state(page, NR_PAGETABLE);
> >          page->cow_pte_owner = NULL;
> > +       atomic_set(&page->cow_pgtable_refcount, 1);
> >          return true;
> >   }
> > 
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 5dcbd7f6c361..984d81e47d53 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -221,6 +221,7 @@ struct page {
> >   #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
> >          int _last_cpupid;
> >   #endif
> > +       atomic_t cow_pgtable_refcount; /* COW page table */
> >          pmd_t *cow_pte_owner; /* cow pte: pmd */
> >   } _struct_page_alignment;
> > 
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index faca57af332e..33c01fec7b92 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -604,6 +604,33 @@ static inline bool cow_pte_owner_is_same(pmd_t *pmd, pmd_t *owner)
> >                  true : false;
> >   }
> > 
> > +extern void cow_pte_fallback(struct vm_area_struct *vma, pmd_t *pmd,
> > +               unsigned long addr);
> 
> 'extern' keyword is pointless for fonction prototypes. No new ones 
> should added.

I see.
It totally does not need the extern to the non-static function.

> > +
> > +static inline int pmd_get_pte(pmd_t *pmd)
> > +{
> > +       return atomic_inc_return(&pmd_page(*pmd)->cow_pgtable_refcount);
> > +}
> > +
> > +/* If the COW PTE page->cow_pgtable_refcount is 1, instead of decreasing the
> > + * counter, clear write protection of the corresponding PMD entry and reset
> > + * the COW PTE owner to reuse the table.
> > + */
> > +static inline int pmd_put_pte(struct vm_area_struct *vma, pmd_t *pmd,
> > +               unsigned long addr)
> > +{
> > +       if (!atomic_add_unless(&pmd_page(*pmd)->cow_pgtable_refcount, -1, 1)) {
> > +               cow_pte_fallback(vma, pmd, addr);
> > +               return 1;
> > +       }
> > +       return 0;
> 
> I would do something more flat by reverting the test:
> 
> {
> 	if (atomic_add_unless(&pmd_page(*pmd)->cow_pgtable_refcount, -1, 1))
> 		return 0;
> 
> 	cow_pte_fallback(vma, pmd, addr);
> 	return 1;
> }
> 

Thanks!