Re: [PATCH 10/19] thp: PMD splitting without splitting compound page

From: Naoya Horiguchi
Date: Wed Nov 19 2014 - 02:00:01 EST

On Wed, Nov 05, 2014 at 04:49:45PM +0200, Kirill A. Shutemov wrote:
> Current split_huge_page() combines two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> changes split_huge_pmd() implementation to split the given PMD without
> splitting other PMDs this page mapped with or underlying compound page.
> In order to do this we have to get rid of tail page refcounting, which
> uses _mapcount of tail pages. Tail page refcounting is needed to be able
> to split THP page at any point: we always know which of tail pages is
> pinned (i.e. by get_user_pages()) and can distribute page count
> correctly.
> We can avoid this by allowing split_huge_page() to fail if the compound
> page is pinned. This patch removes all infrastructure for tail page
> refcounting and make split_huge_page() to always return -EBUSY. All
> split_huge_page() users already know how to handle its fail. Proper
> implementation will be added later.
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---

> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index 7e70ae968e5f..e4ba17694b6b 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -1022,7 +1022,6 @@ int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
> {
> unsigned long mask;
> unsigned long pte_end;
> - struct page *head, *page, *tail;
> pte_t pte;
> int refs;

This breaks build of powerpc, so you need keep *head and *page as
you do for other architectures.--
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