Re: [patch 12/18] hugetlbfs: support larger than MAX_ORDER

[Andrew Hastings <abh@cray.com>]

npiggin@suse.de wrote:
> This is needed on x86-64 to handle GB pages in hugetlbfs, because it is
> not practical to enlarge MAX_ORDER to 1GB. 

Sorry to ask what is probably a dumb question, but why is it not 
practical to increase MAX_ORDER to 1GB for a 64-bit platform like 
x86-64?  Doing so would make 1GB pages much more practical to use.

> Instead the 1GB pages are only allocated at boot using the bootmem
> allocator using the hugepages=... option.
> 
> These 1G bootmem pages are never freed. In theory it would be possible
> to implement that with some complications, but since it would be a one-way
> street (>= MAX_ORDER pages cannot be allocated later) I decided not to
> currently.
> 
> The >= MAX_ORDER code is not ifdef'ed per architecture. It is not very big
> and the ifdef uglyness seemed not be worth it.
> 
> Known problems: /proc/meminfo and "free" do not display the memory 
> allocated for gb pages in "Total". This is a little confusing for the
> user.
> 
> Signed-off-by: Andi Kleen <ak@suse.de>
> Signed-off-by: Nick Piggin <npiggin@suse.de>
> ---
>  mm/hugetlb.c |   74 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 72 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/mm/hugetlb.c
> ===================================================================
> --- linux-2.6.orig/mm/hugetlb.c
> +++ linux-2.6/mm/hugetlb.c
> @@ -14,6 +14,7 @@
>  #include <linux/mempolicy.h>
>  #include <linux/cpuset.h>
>  #include <linux/mutex.h>
> +#include <linux/bootmem.h>
>  
>  #include <asm/page.h>
>  #include <asm/pgtable.h>
> @@ -160,7 +161,7 @@ static void free_huge_page(struct page *
>  	INIT_LIST_HEAD(&page->lru);
>  
>  	spin_lock(&hugetlb_lock);
> -	if (h->surplus_huge_pages_node[nid]) {
> +	if (h->surplus_huge_pages_node[nid] && h->order < MAX_ORDER) {
>  		update_and_free_page(h, page);
>  		h->surplus_huge_pages--;
>  		h->surplus_huge_pages_node[nid]--;
> @@ -222,6 +223,9 @@ static struct page *alloc_fresh_huge_pag
>  {
>  	struct page *page;
>  
> +	if (h->order >= MAX_ORDER)
> +		return NULL;
> +
>  	page = alloc_pages_node(nid,
>  		htlb_alloc_mask|__GFP_COMP|__GFP_THISNODE|__GFP_NOWARN,
>  		huge_page_order(h));
> @@ -278,6 +282,9 @@ static struct page *alloc_buddy_huge_pag
>  	struct page *page;
>  	unsigned int nid;
>  
> +	if (h->order >= MAX_ORDER)
> +		return NULL;
> +
>  	/*
>  	 * Assume we will successfully allocate the surplus page to
>  	 * prevent racing processes from causing the surplus to exceed
> @@ -444,6 +451,10 @@ static void return_unused_surplus_pages(
>  	/* Uncommit the reservation */
>  	h->resv_huge_pages -= unused_resv_pages;
>  
> +	/* Cannot return gigantic pages currently */
> +	if (h->order >= MAX_ORDER)
> +		return;
> +
>  	nr_pages = min(unused_resv_pages, h->surplus_huge_pages);
>  
>  	while (remaining_iterations-- && nr_pages) {
> @@ -522,6 +533,51 @@ static struct page *alloc_huge_page(stru
>  	return page;
>  }
>  
> +static __initdata LIST_HEAD(huge_boot_pages);
> +
> +struct huge_bm_page {
> +	struct list_head list;
> +	struct hstate *hstate;
> +};
> +
> +static int __init alloc_bm_huge_page(struct hstate *h)
> +{
> +	struct huge_bm_page *m;
> +	int nr_nodes = nodes_weight(node_online_map);
> +
> +	while (nr_nodes) {
> +		m = __alloc_bootmem_node_nopanic(NODE_DATA(h->hugetlb_next_nid),
> +					huge_page_size(h), huge_page_size(h),
> +					0);
> +		if (m)
> +			goto found;
> +		hstate_next_node(h);
> +		nr_nodes--;
> +	}
> +	return 0;
> +
> +found:
> +	BUG_ON((unsigned long)virt_to_phys(m) & (huge_page_size(h) - 1));
> +	/* Put them into a private list first because mem_map is not up yet */
> +	list_add(&m->list, &huge_boot_pages);
> +	m->hstate = h;
> +	return 1;
> +}
> +
> +/* Put bootmem huge pages into the standard lists after mem_map is up */
> +static void __init gather_bootmem_prealloc(void)
> +{
> +	struct huge_bm_page *m;
> +	list_for_each_entry (m, &huge_boot_pages, list) {
> +		struct page *page = virt_to_page(m);
> +		struct hstate *h = m->hstate;
> +		__ClearPageReserved(page);
> +		WARN_ON(page_count(page) != 1);
> +		prep_compound_page(page, h->order);
> +		prep_new_huge_page(h, page);
> +	}
> +}
> +
>  static void __init hugetlb_init_hstate(struct hstate *h)
>  {
>  	unsigned long i;
> @@ -532,7 +588,10 @@ static void __init hugetlb_init_hstate(s
>  	h->hugetlb_next_nid = first_node(node_online_map);
>  
>  	for (i = 0; i < h->max_huge_pages; ++i) {
> -		if (!alloc_fresh_huge_page(h))
> +		if (h->order >= MAX_ORDER) {
> +			if (!alloc_bm_huge_page(h))
> +				break;
> +		} else if (!alloc_fresh_huge_page(h))
>  			break;
>  	}
>  	h->max_huge_pages = h->free_huge_pages = h->nr_huge_pages = i;
> @@ -569,6 +628,8 @@ static int __init hugetlb_init(void)
>  
>  	hugetlb_init_hstates();
>  
> +	gather_bootmem_prealloc();
> +
>  	report_hugepages();
>  
>  	return 0;
> @@ -625,6 +686,9 @@ static void try_to_free_low(struct hstat
>  {
>  	int i;
>  
> +	if (h->order >= MAX_ORDER)
> +		return;
> +
>  	for (i = 0; i < MAX_NUMNODES; ++i) {
>  		struct page *page, *next;
>  		struct list_head *freel = &h->hugepage_freelists[i];
> @@ -654,6 +718,12 @@ set_max_huge_pages(struct hstate *h, uns
>  
>  	*err = 0;
>  
> +	if (h->order >= MAX_ORDER) {
> +		if (count != h->max_huge_pages)
> +			*err = -EINVAL;
> +		return h->max_huge_pages;
> +	}
> +
>  	/*
>  	 * Increase the pool size
>  	 * First take pages out of surplus state.  Then make up the
> 

Acked-by: Andrew Hastings <abh@cray.com>

-Andrew Hastings
  Cray Inc.

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