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

Andrew Morton
2022-08-11T23:07:46Z

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()?

> 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