Re: [PATCH 2/3] mm,thp,rmap: simplify compound page mapcount handling

[Hugh Dickins <hughd@google.com>]

On Sat, 5 Nov 2022, Kirill A. Shutemov wrote:
> On Wed, Nov 02, 2022 at 06:51:38PM -0700, Hugh Dickins wrote:
> 
> Thanks for doing this!
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Thanks!

> 
> And sorry again for PageDoubleMap() :/

It did serve a real purpose, but I always found it hard to live with,
and I'm glad that you're happy it's gone too :)

> 
> Minor nitpick and a question below.
> 
> > @@ -829,12 +829,20 @@ static inline int folio_entire_mapcount(struct folio *folio)
> >  
> >  /*
> >   * Mapcount of compound page as a whole, does not include mapped sub-pages.
> > - *
> > - * Must be called only for compound pages.
> > + * Must be called only on head of compound page.
> >   */
> > -static inline int compound_mapcount(struct page *page)
> > +static inline int head_compound_mapcount(struct page *head)
> >  {
> > -	return folio_entire_mapcount(page_folio(page));
> > +	return atomic_read(compound_mapcount_ptr(head)) + 1;
> > +}
> > +
> > +/*
> > + * Sum of mapcounts of sub-pages, does not include compound mapcount.
> > + * Must be called only on head of compound page.
> > + */
> > +static inline int head_subpages_mapcount(struct page *head)
> > +{
> > +	return atomic_read(subpages_mapcount_ptr(head));
> >  }
> >  
> >  /*
> 
> Any particular reason these two do not take struct folio as an input?
> It would guarantee that it is non-tail page. It will not guarantee
> large-folio, but it is something.

The actual reason is that I first did this work in a pre-folio tree;
and even now I am much more at ease with compound pages than folios.

But when I looked to see if I ought to change them, found that the
only uses are below in this header file, or in __dump_page() or in
free_tail_pages_check() - low-level functions, page-oriented and
obviously on head.  So I wasn't tempted to change them at all.

> 
> > @@ -1265,8 +1288,6 @@ void page_add_new_anon_rmap(struct page *page,
> >  		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> >  		/* increment count (starts at -1) */
> >  		atomic_set(compound_mapcount_ptr(page), 0);
> > -		atomic_set(compound_pincount_ptr(page), 0);
> > -
> 
> It has to be initialized to 0 on allocation, right?

That's right.  I was going to say that I'd commented on this in the
commit message, but no, it looks like I only commented on the instance
in hugepage_add_new_new_anon_rmap() (and added the "increment" comment
line from here to there).

I visited both those functions to add a matching subpages_mapcount
initialization; then realized that the pincount addition had missed
the point, initialization to 0 has already been done, and the
compound_mapcount line is about incrementing from -1 to 0,
not about initializing.

There are similar places in mm/hugetlb.c, where I did add the
subpages_mapcount initialization to the compound_pincount and
compound_mapcount initializations: that's because I'm on shaky ground
with hugetlb page lifecycle, and not so sure of their status there.

Hugh