Re: [PATCH 1/1] hugetlbfs: handle pages higher order than MAX_ORDER

[Mel Gorman <mel@csn.ul.ie>]

On (08/10/08 10:33), Andy Whitcroft didst pronounce:
> When working with hugepages, hugetlbfs assumes that those hugepages
> are smaller than MAX_ORDER.  Specifically it assumes that the mem_map
> is contigious and uses that to optimise access to the elements of the
> mem_map that represent the hugepage.  Gigantic pages (such as 16GB pages
> on powerpc) by definition are of greater order than MAX_ORDER (larger
> than MAX_ORDER_NR_PAGES in size).  This means that we can no longer make
> use of the buddy alloctor guarentees for the contiguity of the mem_map,
> which ensures that the mem_map is at least contigious for maximmally
> aligned areas of MAX_ORDER_NR_PAGES pages.
> 
> This patch adds new mem_map accessors and iterator helpers which handle
> any discontiguity at MAX_ORDER_NR_PAGES boundaries.  It then uses these
> within copy_huge_page, clear_huge_page, and follow_hugetlb_page to allow
> these to handle gigantic pages.
> 
> Signed-off-by: Andy Whitcroft <apw@shadowen.org>

Acked-by: Mel Gorman <mel@csn.ul.ie>

> ---
>  mm/hugetlb.c  |   15 ++++++++++-----
>  mm/internal.h |   28 ++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 67a7119..bb5cf81 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -357,11 +357,12 @@ static void clear_huge_page(struct page *page,
>  			unsigned long addr, unsigned long sz)
>  {
>  	int i;
> +	struct page *p = page;
>  
>  	might_sleep();
> -	for (i = 0; i < sz/PAGE_SIZE; i++) {
> +	for (i = 0; i < sz/PAGE_SIZE; i++, p = mem_map_next(p, page, i)) {
>  		cond_resched();
> -		clear_user_highpage(page + i, addr + i * PAGE_SIZE);
> +		clear_user_highpage(p, addr + i * PAGE_SIZE);
>  	}
>  }
>  
> @@ -370,11 +371,15 @@ static void copy_huge_page(struct page *dst, struct page *src,
>  {
>  	int i;
>  	struct hstate *h = hstate_vma(vma);
> +	struct page *dst_base = dst;
> +	struct page *src_base = src;
>  
>  	might_sleep();
> -	for (i = 0; i < pages_per_huge_page(h); i++) {
> +	for (i = 0; i < pages_per_huge_page(h); i++,
> +				dst = mem_map_next(dst, dst_base, i),
> +				src = mem_map_next(src, src_base, i)) {
>  		cond_resched();
> -		copy_user_highpage(dst + i, src + i, addr + i*PAGE_SIZE, vma);
> +		copy_user_highpage(dst, src, addr + i*PAGE_SIZE, vma);
>  	}
>  }
>  
> @@ -2103,7 +2108,7 @@ int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  same_page:
>  		if (pages) {
>  			get_page(page);
> -			pages[i] = page + pfn_offset;
> +			pages[i] = mem_map_offset(page, pfn_offset);
>  		}
>  
>  		if (vmas)
> diff --git a/mm/internal.h b/mm/internal.h
> index 1f43f74..08b8dea 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -53,6 +53,34 @@ static inline unsigned long page_order(struct page *page)
>  }
>  
>  /*
> + * Return the mem_map entry representing the 'offset' subpage within
> + * the maximally aligned gigantic page 'base'.  Handle any discontiguity
> + * in the mem_map at MAX_ORDER_NR_PAGES boundaries.
> + */
> +static inline struct page *mem_map_offset(struct page *base, int offset)
> +{
> +	if (unlikely(offset >= MAX_ORDER_NR_PAGES))
> +		return pfn_to_page(page_to_pfn(base) + offset);
> +	return base + offset;
> +}
> +
> +/*
> + * Iterator over all subpages withing the maximally aligned gigantic
> + * page 'base'.  Handle any discontiguity in the mem_map.
> + */
> +static inline struct page *mem_map_next(struct page *iter,
> +						struct page *base, int offset)
> +{
> +	if (unlikely((offset & (MAX_ORDER_NR_PAGES - 1)) == 0)) {
> +		unsigned long pfn = page_to_pfn(base) + offset;
> +		if (!pfn_valid(pfn))
> +			return NULL;
> +		return pfn_to_page(pfn);
> +	}
> +	return iter + 1;
> +}
> +
> +/*
>   * FLATMEM and DISCONTIGMEM configurations use alloc_bootmem_node,
>   * so all functions starting at paging_init should be marked __init
>   * in those cases. SPARSEMEM, however, allows for memory hotplug,
> -- 
> 1.6.0.1.451.gc8d31
> 

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