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References: <20220801173155.92008-1-ryncsn@gmail.com> <20220811160739.ea6b0f9c2912317c49287cda@linux-foundation.org>
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From:   Kairui Song <ryncsn@gmail.com>
Date:   Fri, 12 Aug 2022 12:49:29 +0800
Message-ID: <CAMgjq7CebYhwgUzBV9tGENEfVMxPMS4qVXvRx=sVQOEAUbq_mA@mail.gmail.com>
Subject: Re: [PATCH] mm/util: reduce stack usage of folio_mapcount
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Matthew Wilcox <willy@infradead.org>
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Andrew Morton <akpm@linux-foundation.org> =E4=BA=8E2022=E5=B9=B48=E6=9C=881=
2=E6=97=A5=E5=91=A8=E4=BA=94 07:07=E5=86=99=E9=81=93=EF=BC=9A
>
> On Tue,  2 Aug 2022 01:31:55 +0800 Kairui Song <ryncsn@gmail.com> wrote:
>
> > From: Kairui Song <kasong@tencent.com>
> >
> > folio_entire_mapcount will call PageHeadHuge which is a function call,
> > and blocks the compiler from recognizing this redundant load.
>
> Did you mean folio_test_hugetlb() rather than folio_entire_mapcount()?

Thanks for checking out this patch, and Yes, it's folio_test_hugetlb,
my mistake...

>
>
> > After rearranging the code, stack usage is dropped from 32 to 24, and
> > the function size is smaller (tested on GCC 12):
> >
> > Before:
> > Stack usage:
> > mm/util.c:845:5:folio_mapcount  32      static
> > Size:
> > 0000000000000ea0 00000000000000c7 T folio_mapcount
> >
> > After:
> > Stack usage:
> > mm/util.c:845:5:folio_mapcount  24      static
> > Size:
> > 0000000000000ea0 00000000000000b0 T folio_mapcount
> >
> > ...
> >
> > @@ -850,10 +850,10 @@ int folio_mapcount(struct folio *folio)
> >               return atomic_read(&folio->_mapcount) + 1;
> >
> >       compound =3D folio_entire_mapcount(folio);
> > -     nr =3D folio_nr_pages(folio);
> >       if (folio_test_hugetlb(folio))
> >               return compound;
> >       ret =3D compound;
> > +     nr =3D folio_nr_pages(folio);
> >       for (i =3D 0; i < nr; i++)
> >               ret +=3D atomic_read(&folio_page(folio, i)->_mapcount) + =
1;
> >       /* File pages has compound_mapcount included in _mapcount */
> > --
> > 2.35.2

Is the rest of the patch a valid fix? Should I send V2?