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SubjectRe: [PATCH 1/8] mm: introduce PageHuge() for testing huge/gigantic pages
Sorry to join the game so late.

On Fri, May 08, 2009 at 06:53:21PM +0800, Wu Fengguang wrote:
> Introduce PageHuge(), which identifies huge/gigantic pages
> by their dedicated compound destructor functions.
>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> include/linux/mm.h | 24 ++++++++++++++++++++++++
> mm/hugetlb.c | 2 +-
> mm/page_alloc.c | 11 ++++++++++-
> 3 files changed, 35 insertions(+), 2 deletions(-)
>
> --- linux.orig/mm/page_alloc.c
> +++ linux/mm/page_alloc.c
> @@ -299,13 +299,22 @@ void prep_compound_page(struct page *pag
> }
>
> #ifdef CONFIG_HUGETLBFS
> +/*
> + * This (duplicated) destructor function distinguishes gigantic pages from
> + * normal compound pages.
> + */
> +void free_gigantic_page(struct page *page)
> +{
> + __free_pages_ok(page, compound_order(page));
> +}
> +
> void prep_compound_gigantic_page(struct page *page, unsigned long order)
> {
> int i;
> int nr_pages = 1 << order;
> struct page *p = page + 1;
>
> - set_compound_page_dtor(page, free_compound_page);
> + set_compound_page_dtor(page, free_gigantic_page);
> set_compound_order(page, order);

This made me raise an eyebrow. gigantic pages can never end up back in the
page allocator. It should cause bugs all over the place so I looked closer
and this free_gigantic_page() looks unnecessary.

This is what happens for gigantic pages at boot-time

gather_bootmem_prealloc() called at boot-time to gather gigantic pages
-> Find the boot allocated pages and call prep_compound_huge_page()
-> For gigantic pages, call prep_compound_gigantic_page(), sets destructor to free_compound_page()
-> Call prep_new_huge_page(), sets destructor to free_huge_page()

So, free_gigantic_page() should never used as such in reality and you can
just check free_huge_page(). If a gigantic page was really freed that way,
it would be really bad.

Does that make sense?


> __SetPageHead(page);
> for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
> --- linux.orig/mm/hugetlb.c
> +++ linux/mm/hugetlb.c
> @@ -550,7 +550,7 @@ struct hstate *size_to_hstate(unsigned l
> return NULL;
> }
>
> -static void free_huge_page(struct page *page)
> +void free_huge_page(struct page *page)
> {
> /*
> * Can't pass hstate in here because it is called from the
> --- linux.orig/include/linux/mm.h
> +++ linux/include/linux/mm.h
> @@ -355,6 +355,30 @@ static inline void set_compound_order(st
> page[1].lru.prev = (void *)order;
> }
>
> +#ifdef CONFIG_HUGETLBFS
> +void free_huge_page(struct page *page);
> +void free_gigantic_page(struct page *page);
> +
> +static inline int PageHuge(struct page *page)
> +{
> + compound_page_dtor *dtor;
> +
> + if (!PageCompound(page))
> + return 0;
> +
> + page = compound_head(page);
> + dtor = get_compound_page_dtor(page);
> +
> + return dtor == free_huge_page ||
> + dtor == free_gigantic_page;
> +}
> +#else
> +static inline int PageHuge(struct page *page)
> +{
> + return 0;
> +}
> +#endif

That is fairly hefty function to be inline and it exports free_huge_page
and free_gigantic_page. The latter of which is dead code and the former
which was previously a static function.

At least make PageHuge a non-inlined function contained in mm/hugetlb.c and
expose it via mm/internal.h if possible or include/linux/hugetlb.h otherwise.

> +
> /*
> * Multiple processes may "see" the same page. E.g. for untouched
> * mappings of /dev/null, all processes see the same page full of
>
> --
>
> --
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>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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