Re: [PATCH] powerpc/mm: Use refcount_t for refcount

[Chuhong Yuan]

On Fri, Aug 9, 2019 at 8:36 PM Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
>
> Chuhong Yuan <hslester96@xxxxxxxxx> writes:
> > Reference counters are preferred to use refcount_t instead of
> > atomic_t.
> > This is because the implementation of refcount_t can prevent
> > overflows and detect possible use-after-free.
> > So convert atomic_t ref counters to refcount_t.
> >
> > Signed-off-by: Chuhong Yuan <hslester96@xxxxxxxxx>
>
> Thanks.
>
> We don't have a fast implementation of refcount_t, so I'm worried this
> could cause a measurable performance regression.
>
> Did you benchmark it at all?
>

I did not benchmark it and I don't have the testing environment...

> cheers
>
> > diff --git a/arch/powerpc/mm/book3s64/mmu_context.c b/arch/powerpc/mm/book3s64/mmu_context.c
> > index 2d0cb5ba9a47..f836fd5a6abc 100644
> > --- a/arch/powerpc/mm/book3s64/mmu_context.c
> > +++ b/arch/powerpc/mm/book3s64/mmu_context.c
> > @@ -231,7 +231,7 @@ static void pmd_frag_destroy(void *pmd_frag)
> >       /* drop all the pending references */
> >       count = ((unsigned long)pmd_frag & ~PAGE_MASK) >> PMD_FRAG_SIZE_SHIFT;
> >       /* We allow PTE_FRAG_NR fragments from a PTE page */
> > -     if (atomic_sub_and_test(PMD_FRAG_NR - count, &page->pt_frag_refcount)) {
> > +     if (refcount_sub_and_test(PMD_FRAG_NR - count, &page->pt_frag_refcount)) {
> >               pgtable_pmd_page_dtor(page);
> >               __free_page(page);
> >       }
> > diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> > index 7d0e0d0d22c4..40056896ce4e 100644
> > --- a/arch/powerpc/mm/book3s64/pgtable.c
> > +++ b/arch/powerpc/mm/book3s64/pgtable.c
> > @@ -277,7 +277,7 @@ static pmd_t *__alloc_for_pmdcache(struct mm_struct *mm)
> >               return NULL;
> >       }
> >
> > -     atomic_set(&page->pt_frag_refcount, 1);
> > +     refcount_set(&page->pt_frag_refcount, 1);
> >
> >       ret = page_address(page);
> >       /*
> > @@ -294,7 +294,7 @@ static pmd_t *__alloc_for_pmdcache(struct mm_struct *mm)
> >        * count.
> >        */
> >       if (likely(!mm->context.pmd_frag)) {
> > -             atomic_set(&page->pt_frag_refcount, PMD_FRAG_NR);
> > +             refcount_set(&page->pt_frag_refcount, PMD_FRAG_NR);
> >               mm->context.pmd_frag = ret + PMD_FRAG_SIZE;
> >       }
> >       spin_unlock(&mm->page_table_lock);
> > @@ -317,8 +317,7 @@ void pmd_fragment_free(unsigned long *pmd)
> >  {
> >       struct page *page = virt_to_page(pmd);
> >
> > -     BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
> > -     if (atomic_dec_and_test(&page->pt_frag_refcount)) {
> > +     if (refcount_dec_and_test(&page->pt_frag_refcount)) {
> >               pgtable_pmd_page_dtor(page);
> >               __free_page(page);
> >       }
> > diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
> > index a7b05214760c..4ef8231b677f 100644
> > --- a/arch/powerpc/mm/pgtable-frag.c
> > +++ b/arch/powerpc/mm/pgtable-frag.c
> > @@ -24,7 +24,7 @@ void pte_frag_destroy(void *pte_frag)
> >       /* drop all the pending references */
> >       count = ((unsigned long)pte_frag & ~PAGE_MASK) >> PTE_FRAG_SIZE_SHIFT;
> >       /* We allow PTE_FRAG_NR fragments from a PTE page */
> > -     if (atomic_sub_and_test(PTE_FRAG_NR - count, &page->pt_frag_refcount)) {
> > +     if (refcount_sub_and_test(PTE_FRAG_NR - count, &page->pt_frag_refcount)) {
> >               pgtable_page_dtor(page);
> >               __free_page(page);
> >       }
> > @@ -71,7 +71,7 @@ static pte_t *__alloc_for_ptecache(struct mm_struct *mm, int kernel)
> >                       return NULL;
> >       }
> >
> > -     atomic_set(&page->pt_frag_refcount, 1);
> > +     refcount_set(&page->pt_frag_refcount, 1);
> >
> >       ret = page_address(page);
> >       /*
> > @@ -87,7 +87,7 @@ static pte_t *__alloc_for_ptecache(struct mm_struct *mm, int kernel)
> >        * count.
> >        */
> >       if (likely(!pte_frag_get(&mm->context))) {
> > -             atomic_set(&page->pt_frag_refcount, PTE_FRAG_NR);
> > +             refcount_set(&page->pt_frag_refcount, PTE_FRAG_NR);
> >               pte_frag_set(&mm->context, ret + PTE_FRAG_SIZE);
> >       }
> >       spin_unlock(&mm->page_table_lock);
> > @@ -110,8 +110,7 @@ void pte_fragment_free(unsigned long *table, int kernel)
> >  {
> >       struct page *page = virt_to_page(table);
> >
> > -     BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
> > -     if (atomic_dec_and_test(&page->pt_frag_refcount)) {
> > +     if (refcount_dec_and_test(&page->pt_frag_refcount)) {
> >               if (!kernel)
> >                       pgtable_page_dtor(page);
> >               __free_page(page);
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 3a37a89eb7a7..7fe23a3faf95 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -14,6 +14,7 @@
> >  #include <linux/uprobes.h>
> >  #include <linux/page-flags-layout.h>
> >  #include <linux/workqueue.h>
> > +#include <linux/refcount.h>
> >
> >  #include <asm/mmu.h>
> >
> > @@ -147,7 +148,7 @@ struct page {
> >                       unsigned long _pt_pad_2;        /* mapping */
> >                       union {
> >                               struct mm_struct *pt_mm; /* x86 pgds only */
> > -                             atomic_t pt_frag_refcount; /* powerpc */
> > +                             refcount_t pt_frag_refcount; /* powerpc */
> >                       };
> >  #if ALLOC_SPLIT_PTLOCKS
> >                       spinlock_t *ptl;
> > --
> > 2.20.1