Re: [PATCH 2/3] mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped

[Hugh Dickins <hughd@google.com>]

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@linux.intel.com>

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