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

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Date:   Tue, 22 Nov 2022 01:13:39 -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>,
        Yu Zhao <yuzhao@...gle.com>,
        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 fix] mm,thp,rmap: subpages_mapcount of PTE-mapped
 subpages: fix

On Mon, 21 Nov 2022, Kirill A. Shutemov wrote:
> On Fri, Nov 18, 2022 at 05:35:05PM -0800, Hugh Dickins wrote:
...
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1281,7 +1281,7 @@ void page_add_anon_rmap(struct page *page,
> >  	struct compound_mapcounts mapcounts;
> >  	int nr = 0, nr_pmdmapped = 0;
> >  	bool compound = flags & RMAP_COMPOUND;
> > -	bool first;
> > +	bool first = true;
> >  
> >  	if (unlikely(PageKsm(page)))
> >  		lock_page_memcg(page);
> 
> Other option is to drop PageTransHuge() check that you already claim to be
> redundant.
> 
> Or have else BUG() to catch cases where the helper called with
> compound=true on non-THP page.

I'm sticking with the "first = true".  I did receive a report of bloating
some tiny config a little, on the very first series, so I've been on guard
since then about adding THP code where the optimizer cannot see to remove
it: so do want to keep the PageTransHuge check in there.  Could be done
in other ways, yes, but I'd ended up feeling this was the best compromise.

Hugh