Date:	Tue, 19 Mar 2013 23:55:33 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/9] memory-hotplug: enable memory hotplug to handle
 hugepage

On Mon, Mar 18, 2013 at 05:07:37PM +0100, Michal Hocko wrote:
> On Thu 21-02-13 14:41:47, Naoya Horiguchi wrote:
> > Currently we can't offline memory blocks which contain hugepages because
> > a hugepage is considered as an unmovable page. But now with this patch
> > series, a hugepage has become movable, so by using hugepage migration we
> > can offline such memory blocks.
> > 
> > What's different from other users of hugepage migration is that we need
> > to decompose all the hugepages inside the target memory block into free
> > buddy pages after hugepage migration, because otherwise free hugepages
> > remaining in the memory block intervene the memory offlining.
> > For this reason we introduce new functions dissolve_free_huge_page() and
> > dissolve_free_huge_pages().
> > 
> > Other than that, what this patch does is straightforwardly to add hugepage
> > migration code, that is, adding hugepage code to the functions which scan
> > over pfn and collect hugepages to be migrated, and adding a hugepage
> > allocation function to alloc_migrate_target().
> > 
> > As for larger hugepages (1GB for x86_64), it's not easy to do hotremove
> > over them because it's larger than memory block. So we now simply leave
> > it to fail as it is.
> 
> What we could do is to check whether there is a free gb huge page on
> other node and migrate there.

Correct, and 1GB page migration needs more code in migration core code
(mainly it's related to migration entry in pud) and enough testing,
so I want to do it in separate patchset.

> > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> > ---
> >  include/linux/hugetlb.h |  8 ++++++++
> >  mm/hugetlb.c            | 43 +++++++++++++++++++++++++++++++++++++++++
> >  mm/memory_hotplug.c     | 51 ++++++++++++++++++++++++++++++++++++++++---------
> >  mm/migrate.c            | 12 +++++++++++-
> >  mm/page_alloc.c         | 12 ++++++++++++
> >  mm/page_isolation.c     |  5 +++++
> >  6 files changed, 121 insertions(+), 10 deletions(-)
> > 
> > diff --git v3.8.orig/include/linux/hugetlb.h v3.8/include/linux/hugetlb.h
> > index 86a4d78..e33f07f 100644
> > --- v3.8.orig/include/linux/hugetlb.h
> > +++ v3.8/include/linux/hugetlb.h
> > @@ -70,6 +70,7 @@ int dequeue_hwpoisoned_huge_page(struct page *page);
> >  void putback_active_hugepage(struct page *page);
> >  void putback_active_hugepages(struct list_head *l);
> >  void migrate_hugepage_add(struct page *page, struct list_head *list);
> > +int is_hugepage_movable(struct page *page);
> >  void copy_huge_page(struct page *dst, struct page *src);
> >  
> >  extern unsigned long hugepages_treat_as_movable;
> > @@ -136,6 +137,7 @@ static inline int dequeue_hwpoisoned_huge_page(struct page *page)
> >  #define putback_active_hugepage(p) 0
> >  #define putback_active_hugepages(l) 0
> >  #define migrate_hugepage_add(p, l) 0
> > +#define is_hugepage_movable(x) 0
> >  static inline void copy_huge_page(struct page *dst, struct page *src)
> >  {
> >  }
> > @@ -358,6 +360,10 @@ static inline int hstate_index(struct hstate *h)
> >  	return h - hstates;
> >  }
> >  
> > +extern void dissolve_free_huge_page(struct page *page);
> > +extern void dissolve_free_huge_pages(unsigned long start_pfn,
> > +				     unsigned long end_pfn);
> > +
> >  #else
> >  struct hstate {};
> >  #define alloc_huge_page(v, a, r) NULL
> > @@ -378,6 +384,8 @@ static inline unsigned int pages_per_huge_page(struct hstate *h)
> >  }
> >  #define hstate_index_to_shift(index) 0
> >  #define hstate_index(h) 0
> > +#define dissolve_free_huge_page(p) 0
> > +#define dissolve_free_huge_pages(s, e) 0
> >  #endif
> >  
> >  #endif /* _LINUX_HUGETLB_H */
> > diff --git v3.8.orig/mm/hugetlb.c v3.8/mm/hugetlb.c
> > index ccf9995..c28e6c9 100644
> > --- v3.8.orig/mm/hugetlb.c
> > +++ v3.8/mm/hugetlb.c
> > @@ -843,6 +843,30 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> >  	return ret;
> >  }
> >  
> > +/* Dissolve a given free hugepage into free pages. */
> > +void dissolve_free_huge_page(struct page *page)
> > +{
> > +	if (PageHuge(page) && !page_count(page)) {
> 
> Could you clarify why you are cheking page_count here? I assume it is to
> make sure the page is free but what prevents it being increased before
> you take hugetlb_lock?

There's nothing to prevent it, so it's not safe to check refcount outside
hugetlb_lock.

> > +		struct hstate *h = page_hstate(page);
> > +		int nid = page_to_nid(page);
> > +		spin_lock(&hugetlb_lock);
> > +		list_del(&page->lru);
> > +		h->free_huge_pages--;
> > +		h->free_huge_pages_node[nid]--;
> > +		update_and_free_page(h, page);
> > +		spin_unlock(&hugetlb_lock);
> > +	}
> > +}
> > +
> > +/* Dissolve free hugepages in a given pfn range. Used by memory hotplug. */
> > +void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
> > +{
> > +	unsigned long pfn;
> > +	unsigned int step = 1 << (HUGETLB_PAGE_ORDER);
> 
> hugetlb pages could be present in different sizes so this doesn't work
> in general. You need to to get order from page_hstate.

OK.

> > +	for (pfn = start_pfn; pfn < end_pfn; pfn += step)
> > +		dissolve_free_huge_page(pfn_to_page(pfn));
> > +}
> > +
> >  static struct page *alloc_buddy_huge_page(struct hstate *h, int nid)
> >  {
> >  	struct page *page;
> > @@ -3158,6 +3182,25 @@ static int is_hugepage_on_freelist(struct page *hpage)
> >  	return 0;
> >  }
> >  
> > +/* Returns true for head pages of in-use hugepages, otherwise returns false. */
> > +int is_hugepage_movable(struct page *hpage)
> > +{
> > +	struct page *page;
> > +	struct page *tmp;
> > +	struct hstate *h = page_hstate(hpage);
> > +	int ret = 0;
> > +
> > +	VM_BUG_ON(!PageHuge(hpage));
> > +	if (PageTail(hpage))
> > +		return 0;
> > +	spin_lock(&hugetlb_lock);
> > +	list_for_each_entry_safe(page, tmp, &h->hugepage_activelist, lru)
> > +		if (page == hpage)
> > +			ret = 1;
> > +	spin_unlock(&hugetlb_lock);
> > +	return ret;
> > +}
> > +
> >  /*
> >   * This function is called from memory failure code.
> >   * Assume the caller holds page lock of the head page.
> > diff --git v3.8.orig/mm/memory_hotplug.c v3.8/mm/memory_hotplug.c
> > index d04ed87..6418de2 100644
> > --- v3.8.orig/mm/memory_hotplug.c
> > +++ v3.8/mm/memory_hotplug.c
> > @@ -29,6 +29,7 @@
> >  #include <linux/suspend.h>
> >  #include <linux/mm_inline.h>
> >  #include <linux/firmware-map.h>
> > +#include <linux/hugetlb.h>
> >  
> >  #include <asm/tlbflush.h>
> >  
> > @@ -985,10 +986,12 @@ static int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
> >  }
> >  
> >  /*
> > - * Scanning pfn is much easier than scanning lru list.
> > - * Scan pfn from start to end and Find LRU page.
> > + * Scan pfn range [start,end) to find movable/migratable pages (LRU pages
> > + * and hugepages). We scan pfn because it's much easier than scanning over
> > + * linked list. This function returns the pfn of the first found movable
> > + * page if it's found, otherwise 0.
> >   */
> > -static unsigned long scan_lru_pages(unsigned long start, unsigned long end)
> > +static unsigned long scan_movable_pages(unsigned long start, unsigned long end)
> >  {
> >  	unsigned long pfn;
> >  	struct page *page;
> > @@ -997,6 +1000,12 @@ static unsigned long scan_lru_pages(unsigned long start, unsigned long end)
> >  			page = pfn_to_page(pfn);
> >  			if (PageLRU(page))
> >  				return pfn;
> > +			if (PageHuge(page)) {
> > +				if (is_hugepage_movable(page))
> > +					return pfn;
> > +				else
> > +					pfn += (1 << compound_order(page)) - 1;
> > +			}
> 
> scan_lru_pages's name gets really confusing after this change because
> hugetlb pages are not on the LRU. Maybe it would be good to rename it.

Yes, and that's done in right above chunk.

> 
> >  		}
> >  	}
> >  	return 0;
> > @@ -1019,6 +1028,30 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
> >  		page = pfn_to_page(pfn);
> >  		if (!get_page_unless_zero(page))
> >  			continue;
> 
> All tail pages have 0 reference count (according to prep_compound_page)
> so they would be skipped anyway. This makes the below pfn tweaks
> pointless.

I was totally mistaken about what we should do here, sorry. If we call
do_migrate_range() for 1GB hugepage, we should return with error (maybe -EBUSY)
instead of just skipping it, otherwise the caller __offline_pages() repeats
'goto repeat' until timeout. In order to do that, we had better insert
if(PageHuge) block before getting refcount. And ...

> > +		if (PageHuge(page)) {
> > +			/*
> > +			 * Larger hugepage (1GB for x86_64) is larger than
> > +			 * memory block, so pfn scan can start at the tail
> > +			 * page of larger hugepage. In such case,
> > +			 * we simply skip the hugepage and move the cursor
> > +			 * to the last tail page.
> > +			 */
> > +			if (PageTail(page)) {
> > +				struct page *head = compound_head(page);
> > +				pfn = page_to_pfn(head) +
> > +					(1 << compound_order(head)) - 1;
> > +				put_page(page);
> > +				continue;
> > +			}
> > +			pfn = (1 << compound_order(page)) - 1;
> > +			if (huge_page_size(page_hstate(page)) != PMD_SIZE) {
> > +				put_page(page);
> > +				continue;
> > +			}
> 
> There might be other hugepage sizes which fit into memblock so this test
> doesn't seem right.

yes, so compound_order(head) > PFN_SECTION_SHIFT would be better.

I'll replace this chunk with the following if I don't get any other
suggestion.

@@ -1017,6 +1026,21 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 		if (!pfn_valid(pfn))
 			continue;
 		page = pfn_to_page(pfn);
+
+		if (PageHuge(page)) {
+			struct page *head = compound_head(page);
+			pfn = page_to_pfn(head) + (1 << compound_order(head)) - 1;
+			if (compound_order(head) > PFN_SECTION_SHIFT) {
+				ret = -EBUSY;
+				break;
+			}
+			if (!get_page_unless_zero(page))
+				continue;
+			list_move_tail(&head->lru, &source);
+			move_pages -= 1 << compound_order(head);
+			continue;
+		}
+
 		if (!get_page_unless_zero(page))
 			continue;
 		/*

Thanks,
Naoya
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