On Mon, 21 Nov 2022, Kirill A. Shutemov wrote:
> On Fri, Nov 18, 2022 at 01:14:17AM -0800, Hugh Dickins wrote:
> > Can the lock_compound_mapcount() bit_spin_lock apparatus be removed now?
> > Yes. Not by atomic64_t or cmpxchg games, those get difficult on 32-bit;
> > but if we slightly abuse subpages_mapcount by additionally demanding that
> > one bit be set there when the compound page is PMD-mapped, then a cascade
> > of two atomic ops is able to maintain the stats without bit_spin_lock.
>
> Yay! New home for PageDoubleMap()! :P
:) You only asked for one bit for PageDoubleMap, I've been greedier;
so it's not surprising if it has worked out better now.
...
> Jokes aside, looks neat.
>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Thanks; but I'm very glad that Linus expressed his dissatisfaction
with the first implementation, this one does feel much better.
>
> As always few minor nits below.
...
> > @@ -893,8 +902,12 @@ static inline int total_mapcount(struct page *page)
> >
> > static inline bool folio_large_is_mapped(struct folio *folio)
> > {
> > - return atomic_read(folio_mapcount_ptr(folio)) +
> > - atomic_read(folio_subpages_mapcount_ptr(folio)) >= 0;
> > + /*
> > + * Reading folio_mapcount_ptr() below could be omitted if hugetlb
> > + * participated in incrementing subpages_mapcount when compound mapped.
> > + */
> > + return atomic_read(folio_mapcount_ptr(folio)) >= 0 ||
> > + atomic_read(folio_subpages_mapcount_ptr(folio)) > 0;
>
> Maybe check folio_subpages_mapcount_ptr() first? It would avoid
> folio_mapcount_ptr() read for everything, but hugetlb.
Okay: I'm not convinced, but don't mind switching those around: done.
> > --- a/mm/debug.c
> > +++ b/mm/debug.c
> > @@ -97,7 +97,7 @@ static void __dump_page(struct page *page)
> > pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d compound_pincount:%d\n",
> > head, compound_order(head),
> > head_compound_mapcount(head),
> > - head_subpages_mapcount(head),
> > + head_subpages_mapcount(head) & SUBPAGES_MAPPED,
>
> Looks like applying the SUBPAGES_MAPPED mask belong to the
> head_subpages_mapcount() helper, not to the caller.
Yes, that would be more consistent, helper function doing the massage.
Done. __dump_page() then remains unchanged, but free_tail_pages_check()
uses subpages_mapcount_ptr(head_page) to check the whole field is zero.
v2 coming up - thanks.
Hugh
