linux-kernel - Re: [PATCH 1/3] mm,thp,rmap: subpages

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Message-ID: <efa711e6-3ec2-f297-c146-577a88894c66@google.com>
Date:   Tue, 22 Nov 2022 01:03:42 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>, Peter Xu <peterx@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Mina Almasry <almasrymina@...gle.com>,
        James Houghton <jthoughton@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/3] mm,thp,rmap: subpages_mapcount of PTE-mapped
 subpages

On Mon, 21 Nov 2022, Kirill A. Shutemov wrote:
> On Fri, Nov 18, 2022 at 01:12:03AM -0800, Hugh Dickins wrote:
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

Thanks a lot for all these, Kirill.

> 
> Few minor nitpicks below.
> 
...
> >  static inline void page_dup_file_rmap(struct page *page, bool compound)
> >  {
> > -	if (PageCompound(page))
> > -		page_dup_compound_rmap(page, compound);
> > -	else
> > +	if (likely(!compound /* page is mapped by PTE */))
> 
> I'm not a fan of this kind of comments.
> 
> Maybe move above the line (here and below)?

Okay, done throughout.  I wouldn't have added those comments, but Linus
had another "simmering hatred", of the "compound" arguments: he found
them very confusing.

The real fix is to rename them, probably to pmd_mapped; or better, pass
down an int nr_pages as he suggested; but I'm wary of the HPAGE_NR_PAGES
build bug, and it wants to be propagated through various other files
(headers and mlock.c, maybe more) - not a cleanup to get into now.

> 
> >  		atomic_inc(&page->_mapcount);
> > +	else
> > +		page_dup_compound_rmap(page);
> >  }
...
> > @@ -1176,20 +1157,16 @@ void page_dup_compound_rmap(struct page *page, bool compound)
> >  	 * Note that hugetlb does not call page_add_file_rmap():
> >  	 * here is where hugetlb shared page mapcount is raised.
> >  	 */
> > -	if (PageHuge(page)) {
> > -		atomic_inc(compound_mapcount_ptr(page));
> > -		return;
> > -	}
> > +	if (PageHuge(head)) {
> > +		atomic_inc(compound_mapcount_ptr(head));
> >  
> 
> Remove the newline?

It was intentional there, I thought it was easier to read that way;
but since this gets reverted in the next patch, I've no reason to
fight over it - removed.

Hugh