Re: [PATCH] mm/util: reduce stack usage of folio_mapcount

Kairui Song
2022-08-12T04:49:47Z

Andrew Morton <akpm@linux-foundation.org> 于2022年8月12日周五 07:07写道：
>
> 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 = folio_entire_mapcount(folio);
> > - nr = folio_nr_pages(folio);
> > if (folio_test_hugetlb(folio))
> > return compound;
> > ret = compound;
> > + nr = folio_nr_pages(folio);
> > for (i = 0; i < nr; i++)
> > ret += 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?