Re: [PATCH v4 12/17] mm: remember exclusively mapped anonymous pages with PG_anon_exclusive

[David Hildenbrand]

On 06.12.22 10:37, Miaohe Lin wrote:
> On 2022/12/6 16:43, David Hildenbrand wrote:
> > > >
> > >
> > > Hi David, sorry for the late respond and a possible inconsequential question. :)
> >
> > Better late than never! Thanks for the review, independently at which time it happens :)
> >
> > > <snip>
> > >
> > > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > > index 7a71ed679853..5add8bbd47cd 100644
> > > > --- a/mm/hugetlb.c
> > > > +++ b/mm/hugetlb.c
> > > > @@ -4772,7 +4772,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> > > >                        is_hugetlb_entry_hwpoisoned(entry))) {
> > > >                swp_entry_t swp_entry = pte_to_swp_entry(entry);
> > > >    -            if (is_writable_migration_entry(swp_entry) && cow) {
> > > > +            if (!is_readable_migration_entry(swp_entry) && cow) {
> > > >                    /*
> > > >                     * COW mappings require pages in both
> > > >                     * parent and child to be set to read.
> > > > @@ -5172,6 +5172,8 @@ static vm_fault_t hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
> > > >            set_huge_ptep_writable(vma, haddr, ptep);
> > > >            return 0;
> > > >        }
> > > > +    VM_BUG_ON_PAGE(PageAnon(old_page) && PageAnonExclusive(old_page),
> > > > +               old_page);
> > > >          /*
> > > >         * If the process that created a MAP_PRIVATE mapping is about to
> > > > @@ -6169,12 +6171,17 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> > > >            }
> > > >            if (unlikely(is_hugetlb_entry_migration(pte))) {
> > > >                swp_entry_t entry = pte_to_swp_entry(pte);
> > > > +            struct page *page = pfn_swap_entry_to_page(entry);
> > > >    -            if (is_writable_migration_entry(entry)) {
> > > > +            if (!is_readable_migration_entry(entry)) {
> > >
> > > In hugetlb_change_protection(), is_writable_migration_entry() is changed to !is_readable_migration_entry(),
> > > but
> > >
> > > >                    pte_t newpte;
> > > >    -                entry = make_readable_migration_entry(
> > > > -                            swp_offset(entry));
> > > > +                if (PageAnon(page))
> > > > +                    entry = make_readable_exclusive_migration_entry(
> > > > +                                swp_offset(entry));
> > > > +                else
> > > > +                    entry = make_readable_migration_entry(
> > > > +                                swp_offset(entry));
> > > >                    newpte = swp_entry_to_pte(entry);
> > > >                    set_huge_swap_pte_at(mm, address, ptep,
> > > >                                 newpte, huge_page_size(h));
> > >
> > > <snip>
> > >
> > > > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > > > index b69ce7a7b2b7..56060acdabd3 100644
> > > > --- a/mm/mprotect.c
> > > > +++ b/mm/mprotect.c
> > > > @@ -152,6 +152,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> > > >                pages++;
> > > >            } else if (is_swap_pte(oldpte)) {
> > > >                swp_entry_t entry = pte_to_swp_entry(oldpte);
> > > > +            struct page *page = pfn_swap_entry_to_page(entry);
> > > >                pte_t newpte;
> > > >                  if (is_writable_migration_entry(entry)) {
> > >
> > > In change_pte_range(), is_writable_migration_entry() is not changed to !is_readable_migration_entry().
> >
> > Yes, and also in change_huge_pmd(), is_writable_migration_entry() stays unchanged.
> >
> > > Is this done intentionally? Could you tell me why there's such a difference? I'm confused. It's very
> > > kind of you if you can answer my puzzle.
> >
> > For change protection, the only relevant part is to convert writable -> readable or writable -> readable_exclusive.
> >
> > If an entry is already readable or readable_exclusive, there is nothing to do. The only issues would be when turning a readable one into a readable_exclusive one or a readable_exclusive one into a readable one.
> >
> >
> > In hugetlb_change_protection(), the "!is_readable_migration_entry" could in fact be turned into a "is_writable_migration_entry()". Right now, it would convert writable -> readable or writable -> readable_exclusive AND readable -> readable AND readable_exclusive -> readable_exclusive, which isn't necessary but also shouldn't hurt either.
>
> Many thanks for your explanation. It's really helpful. :)
>
> > So yeah, it's not consistent but shouldn't be problematic. Do you see an issue with that?
>
> No, I don't see any issue with that. I just wonder whether we can change "!is_readable_migration_entry" to "is_writable_migration_entry()" to make code
> more consistent and avoid possible future puzzle. Also we can further remove this harmless unnecessary migration entry conversion. But this should
> be a separate cleanup patch anyway.

Want to send a patch? :)

Thanks!

--
Thanks,

David / dhildenb