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Date:	Fri, 4 Apr 2014 09:30:46 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Cc:	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<mtosatti@...hat.com>, <aarcange@...hat.com>, <mgorman@...e.de>,
	<akpm@...ux-foundation.org>, <andi@...stfloor.org>,
	<davidlohr@...com>, <rientjes@...gle.com>, <yinghai@...nel.org>,
	<riel@...hat.com>
Subject: Re: [PATCH 4/4] hugetlb: add support for gigantic page allocation
 at runtime

On Fri, 4 Apr 2014 12:05:17 +0900
Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com> wrote:

> (2014/04/03 3:08), Luiz Capitulino wrote:
> > HugeTLB is limited to allocating hugepages whose size are less than
> > MAX_ORDER order. This is so because HugeTLB allocates hugepages via
> > the buddy allocator. Gigantic pages (that is, pages whose size is
> > greater than MAX_ORDER order) have to be allocated at boottime.
> > 
> > However, boottime allocation has at least two serious problems. First,
> > it doesn't support NUMA and second, gigantic pages allocated at
> > boottime can't be freed.
> > 
> > This commit solves both issues by adding support for allocating gigantic
> > pages during runtime. It works just like regular sized hugepages,
> > meaning that the interface in sysfs is the same, it supports NUMA,
> > and gigantic pages can be freed.
> > 
> > For example, on x86_64 gigantic pages are 1GB big. To allocate two 1G
> > gigantic pages on node 1, one can do:
> > 
> >   # echo 2 > \
> >     /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages
> > 
> > And to free them later:
> > 
> >   # echo 0 > \
> >     /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages
> > 
> > The one problem with gigantic page allocation at runtime is that it
> > can't be serviced by the buddy allocator. To overcome that problem, this
> > series scans all zones from a node looking for a large enough contiguous
> > region. When one is found, it's allocated by using CMA, that is, we call
> > alloc_contig_range() to do the actual allocation. For example, on x86_64
> > we scan all zones looking for a 1GB contiguous region. When one is found
> > it's allocated by alloc_contig_range().
> > 
> > One expected issue with that approach is that such gigantic contiguous
> > regions tend to vanish as time goes by. The best way to avoid this for
> > now is to make gigantic page allocations very early during boot, say
> > from a init script. Other possible optimization include using compaction,
> > which is supported by CMA but is not explicitly used by this commit.
> > 
> > It's also important to note the following:
> > 
> >   1. My target systems are x86_64 machines, so I have only tested 1GB
> >      pages allocation/release. I did try to make this arch indepedent
> >      and expect it to work on other archs but didn't try it myself
> > 
> >   2. I didn't add support for hugepage overcommit, that is allocating
> >      a gigantic page on demand when
> >     /proc/sys/vm/nr_overcommit_hugepages > 0. The reason is that I don't
> >     think it's reasonable to do the hard and long work required for
> >     allocating a gigantic page at fault time. But it should be simple
> >     to add this if wanted
> > 
> > Signed-off-by: Luiz Capitulino <lcapitulino@...hat.com>
> > ---
> >   arch/x86/include/asm/hugetlb.h |  10 +++
> >   mm/hugetlb.c                   | 177 ++++++++++++++++++++++++++++++++++++++---
> >   2 files changed, 176 insertions(+), 11 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/hugetlb.h b/arch/x86/include/asm/hugetlb.h
> > index a809121..2b262f7 100644
> > --- a/arch/x86/include/asm/hugetlb.h
> > +++ b/arch/x86/include/asm/hugetlb.h
> > @@ -91,6 +91,16 @@ static inline void arch_release_hugepage(struct page *page)
> >   {
> >   }
> >   
> > +static inline int arch_prepare_gigantic_page(struct page *page)
> > +{
> > +	return 0;
> > +}
> > +
> > +static inline void arch_release_gigantic_page(struct page *page)
> > +{
> > +}
> > +
> > +
> >   static inline void arch_clear_hugepage_flags(struct page *page)
> >   {
> >   }
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 2c7a44a..c68515e 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -643,11 +643,159 @@ static int hstate_next_node_to_free(struct hstate *h, nodemask_t *nodes_allowed)
> >   		((node = hstate_next_node_to_free(hs, mask)) || 1);	\
> >   		nr_nodes--)
> >   
> > +#ifdef CONFIG_CMA
> > +static void destroy_compound_gigantic_page(struct page *page,
> > +					unsigned long order)
> > +{
> > +	int i;
> > +	int nr_pages = 1 << order;
> > +	struct page *p = page + 1;
> > +
> > +	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
> > +		__ClearPageTail(p);
> > +		set_page_refcounted(p);
> > +		p->first_page = NULL;
> > +	}
> > +
> > +	set_compound_order(page, 0);
> > +	__ClearPageHead(page);
> > +}
> > +
> > +static void free_gigantic_page(struct page *page, unsigned order)
> > +{
> > +	free_contig_range(page_to_pfn(page), 1 << order);
> > +}
> > +
> > +static int __alloc_gigantic_page(unsigned long start_pfn, unsigned long count)
> > +{
> > +	unsigned long end_pfn = start_pfn + count;
> > +	return alloc_contig_range(start_pfn, end_pfn, MIGRATE_MOVABLE);
> > +}
> > +
> > +static bool pfn_valid_gigantic(unsigned long pfn)
> > +{
> > +	struct page *page;
> > +
> > +	if (!pfn_valid(pfn))
> > +		return false;
> > +
> > +	page = pfn_to_page(pfn);
> > +
> > +	if (PageReserved(page))
> > +		return false;
> > +
> > +	if (page_count(page) > 0)
> > +		return false;
> > +
> > +	return true;
> > +}
> > +
> > +static inline bool pfn_aligned_gigantic(unsigned long pfn, unsigned order)
> > +{
> > +	return IS_ALIGNED((phys_addr_t) pfn << PAGE_SHIFT, PAGE_SIZE << order);
> > +}
> > +
> > +static struct page *alloc_gigantic_page(int nid, unsigned order)
> > +{
> > +	unsigned long ret, i, count, start_pfn, flags;
> > +	unsigned long nr_pages = 1 << order;
> > +	struct zone *z;
> > +
> > +	z = NODE_DATA(nid)->node_zones;
> > +	for (; z - NODE_DATA(nid)->node_zones < MAX_NR_ZONES; z++) {
> > +		spin_lock_irqsave(&z->lock, flags);
> > +		if (z->spanned_pages < nr_pages) {
> > +			spin_unlock_irqrestore(&z->lock, flags);
> > +			continue;
> > +		}
> > +
> > +		/* scan zone 'z' looking for a contiguous 'nr_pages' range */
> > +		count = 0;
> 
> > +		start_pfn = z->zone_start_pfn; /* to silence gcc */
> > +		for (i = z->zone_start_pfn; i < zone_end_pfn(z); i++) {
> 
> This loop is not smart. On our system, one node has serveral TBytes.
> So the maximum loop count is "TBytes/Page size".

Interesting. Would you be willing to test this series on such a
machine?

> First page of gigantic page must be aligned.
> So how about it:
> 
> 		start_pfn = zone_start_pfn aligned gigantic page
> 		for (i = start_pfn; i < zone_end_pfn; i += size of gigantic page) {
> 			if (!pfn_valid_gigantic(i)) {
> 				count = 0;
> 				continue;
> 			}
> 			
> 			...
> 		}

I'm not sure that very loop will work because pfn_valid_gigantic() checks
a single PFN today, but we do have to scan every single PFN on a gigantic
page range.

On the other hand, I think got what you're suggesting. When an unsuitable
PFN is found, we should just skip to the next aligned PFN instead of
keep scanning for nothing (which is what my loop does today). Maybe you're
suggesting pfn_valid_gigantic() should do that?

Anyway, I'll make that change, thank you very much for you review!

> 
> Thanks,
> Yasuaki Ishimatsu
> 
> > +			if (!pfn_valid_gigantic(i)) {
> > +				count = 0;
> > +				continue;
> > +			}
> > +			if (!count) {
> > +				if (!pfn_aligned_gigantic(i, order))
> > +					continue;
> > +				start_pfn = i;
> > +			}
> > +			if (++count == nr_pages) {
> > +				/*
> > +				 * We release the zone lock here because
> > +				 * alloc_contig_range() will also lock the zone
> > +				 * at some point. If there's an allocation
> > +				 * spinning on this lock, it may win the race
> > +				 * and cause alloc_contig_range() to fail...
> > +				 */
> > +				spin_unlock_irqrestore(&z->lock, flags);
> > +				ret = __alloc_gigantic_page(start_pfn, count);
> > +				if (!ret)
> > +					return pfn_to_page(start_pfn);
> > +				count = 0;
> > +				spin_lock_irqsave(&z->lock, flags);
> > +			}
> > +		}
> > +
> > +		spin_unlock_irqrestore(&z->lock, flags);
> > +	}
> > +
> > +	return NULL;
> > +}
> > +
> > +static void prep_new_huge_page(struct hstate *h, struct page *page, int nid);
> > +static void prep_compound_gigantic_page(struct page *page, unsigned long order);
> > +
> > +static struct page *alloc_fresh_gigantic_page_node(struct hstate *h, int nid)
> > +{
> > +	struct page *page;
> > +
> > +	page = alloc_gigantic_page(nid, huge_page_order(h));
> > +	if (page) {
> > +		if (arch_prepare_gigantic_page(page)) {
> > +			free_gigantic_page(page, huge_page_order(h));
> > +			return NULL;
> > +		}
> > +		prep_compound_gigantic_page(page, huge_page_order(h));
> > +		prep_new_huge_page(h, page, nid);
> > +	}
> > +
> > +	return page;
> > +}
> > +
> > +static int alloc_fresh_gigantic_page(struct hstate *h,
> > +				nodemask_t *nodes_allowed)
> > +{
> > +	struct page *page = NULL;
> > +	int nr_nodes, node;
> > +
> > +	for_each_node_mask_to_alloc(h, nr_nodes, node, nodes_allowed) {
> > +		page = alloc_fresh_gigantic_page_node(h, node);
> > +		if (page)
> > +			return 1;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static inline bool gigantic_page_supported(void) { return true; }
> > +#else /* !CONFIG_CMA */
> > +static inline bool gigantic_page_supported(void) { return false; }
> > +static inline void free_gigantic_page(struct page *page, unsigned order) { }
> > +static inline void destroy_compound_gigantic_page(struct page *page,
> > +						unsigned long order) { }
> > +static inline int alloc_fresh_gigantic_page(struct hstate *h,
> > +					nodemask_t *nodes_allowed) { return 0; }
> > +#endif /* CONFIG_CMA */
> > +
> >   static void update_and_free_page(struct hstate *h, struct page *page)
> >   {
> >   	int i;
> >   
> > -	VM_BUG_ON(hstate_is_gigantic(h));
> > +	if (hstate_is_gigantic(h) && !gigantic_page_supported())
> > +		return;
> >   
> >   	h->nr_huge_pages--;
> >   	h->nr_huge_pages_node[page_to_nid(page)]--;
> > @@ -661,8 +809,14 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> >   	VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> >   	set_compound_page_dtor(page, NULL);
> >   	set_page_refcounted(page);
> > -	arch_release_hugepage(page);
> > -	__free_pages(page, huge_page_order(h));
> > +	if (hstate_is_gigantic(h)) {
> > +		arch_release_gigantic_page(page);
> > +		destroy_compound_gigantic_page(page, huge_page_order(h));
> > +		free_gigantic_page(page, huge_page_order(h));
> > +	} else {
> > +		arch_release_hugepage(page);
> > +		__free_pages(page, huge_page_order(h));
> > +	}
> >   }
> >   
> >   struct hstate *size_to_hstate(unsigned long size)
> > @@ -701,7 +855,7 @@ static void free_huge_page(struct page *page)
> >   	if (restore_reserve)
> >   		h->resv_huge_pages++;
> >   
> > -	if (h->surplus_huge_pages_node[nid] && !hstate_is_gigantic(h)) {
> > +	if (h->surplus_huge_pages_node[nid]) {
> >   		/* remove the page from active list */
> >   		list_del(&page->lru);
> >   		update_and_free_page(h, page);
> > @@ -805,9 +959,6 @@ static struct page *alloc_fresh_huge_page_node(struct hstate *h, int nid)
> >   {
> >   	struct page *page;
> >   
> > -	if (hstate_is_gigantic(h))
> > -		return NULL;
> > -
> >   	page = alloc_pages_exact_node(nid,
> >   		htlb_alloc_mask(h)|__GFP_COMP|__GFP_THISNODE|
> >   						__GFP_REPEAT|__GFP_NOWARN,
> > @@ -1452,7 +1603,7 @@ static unsigned long set_max_huge_pages(struct hstate *h, unsigned long count,
> >   {
> >   	unsigned long min_count, ret;
> >   
> > -	if (hstate_is_gigantic(h))
> > +	if (hstate_is_gigantic(h) && !gigantic_page_supported())
> >   		return h->max_huge_pages;
> >   
> >   	/*
> > @@ -1479,7 +1630,11 @@ static unsigned long set_max_huge_pages(struct hstate *h, unsigned long count,
> >   		 * and reducing the surplus.
> >   		 */
> >   		spin_unlock(&hugetlb_lock);
> > -		ret = alloc_fresh_huge_page(h, nodes_allowed);
> > +		if (hstate_is_gigantic(h)) {
> > +			ret = alloc_fresh_gigantic_page(h, nodes_allowed);
> > +		} else {
> > +			ret = alloc_fresh_huge_page(h, nodes_allowed);
> > +		}
> >   		spin_lock(&hugetlb_lock);
> >   		if (!ret)
> >   			goto out;
> > @@ -1578,7 +1733,7 @@ static ssize_t nr_hugepages_store_common(bool obey_mempolicy,
> >   		goto out;
> >   
> >   	h = kobj_to_hstate(kobj, &nid);
> > -	if (hstate_is_gigantic(h)) {
> > +	if (hstate_is_gigantic(h) && !gigantic_page_supported()) {
> >   		err = -EINVAL;
> >   		goto out;
> >   	}
> > @@ -2072,7 +2227,7 @@ static int hugetlb_sysctl_handler_common(bool obey_mempolicy,
> >   
> >   	tmp = h->max_huge_pages;
> >   
> > -	if (write && hstate_is_gigantic(h))
> > +	if (write && hstate_is_gigantic(h) && !gigantic_page_supported())
> >   		return -EINVAL;
> >   
> >   	table->data = &tmp;
> > 
> 
> 

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