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SubjectRe: [PATCH] mm: setup pageblock_order before it's used by sparse
On Sat, 30 Jun 2012 17:07:54 +0800
Jiang Liu <jiang.liu@huawei.com> wrote:

> From: Xishi Qiu <qiuxishi@huawei.com>
>
> On architectures with CONFIG_HUGETLB_PAGE_SIZE_VARIABLE set, such as Itanium,
> pageblock_order is a variable with default value of 0. It's set to the right
> value by set_pageblock_order() in function free_area_init_core().
>
> But pageblock_order may be used by sparse_init() before free_area_init_core()
> is called along path:
> sparse_init()
> ->sparse_early_usemaps_alloc_node()
> ->usemap_size()
> ->SECTION_BLOCKFLAGS_BITS
> ->((1UL << (PFN_SECTION_SHIFT - pageblock_order)) *
> NR_PAGEBLOCK_BITS)
>
> The uninitialized pageblock_size will cause memory wasting because usemap_size()
> returns a much bigger value then it's really needed.
>
> For example, on an Itanium platform,
> sparse_init() pageblock_order=0 usemap_size=24576
> free_area_init_core() before pageblock_order=0, usemap_size=24576
> free_area_init_core() after pageblock_order=12, usemap_size=8
>
> That means 24K memory has been wasted for each section, so fix it by calling
> set_pageblock_order() from sparse_init().
>
> ...
>
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -485,6 +485,9 @@ void __init sparse_init(void)
> struct page **map_map;
> #endif
>
> + /* Setup pageblock_order for HUGETLB_PAGE_SIZE_VARIABLE */
> + set_pageblock_order();
> +
> /*
> * map is using big page (aka 2M in x86 64 bit)
> * usemap is less one page (aka 24 bytes)

It's a bit ugly calling set_pageblock_order() from both sparse_init()
and from free_area_init_core(). Can we find a single place from which
to call it? It looks like here:

--- a/init/main.c~a
+++ a/init/main.c
@@ -514,6 +514,7 @@ asmlinkage void __init start_kernel(void
__stop___param - __start___param,
-1, -1, &unknown_bootoption);

+ set_pageblock_order();
jump_label_init();

/*
would do the trick?

(free_area_init_core is __paging_init and set_pageblock_order() is
__init. I'm too lazy to work out if that's wrong)


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