[PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[Adam Litke]

Fourth Release:
***
This version includes fixes to a few bugs found through review.  Also, the
explicit pool resizing code has been updated to make use of the surplus
counters.  When shrinking the pool below the committed size, the surplus will
be elevated so that committed pages will be released when they are freed.  When
expanding the pool, surplus pages will be consumed before any fresh huge pages
are allocated.
***

In most real-world scenarios, configuring the size of the hugetlb pool
correctly is a difficult task.  If too few pages are allocated to the pool,
applications using MAP_SHARED may fail to mmap() a hugepage region and
applications using MAP_PRIVATE may receive SIGBUS.  Isolating too much memory
in the hugetlb pool means it is not available for other uses, especially those
programs not using huge pages.

The obvious answer is to let the hugetlb pool grow and shrink in response to
the runtime demand for huge pages.  The work Mel Gorman has been doing to
establish a memory zone for movable memory allocations makes dynamically
resizing the hugetlb pool reliable within the limits of that zone.  This patch
series implements dynamic pool resizing for private and shared mappings while
being careful to maintain existing semantics.  Please reply with your comments
and feedback; even just to say whether it would be a useful feature to you.
Thanks.

How it works
============

Upon depletion of the hugetlb pool, rather than reporting an error immediately,
first try and allocate the needed huge pages directly from the buddy allocator.
Care must be taken to avoid unbounded growth of the hugetlb pool, so the
hugetlb filesystem quota is used to limit overall pool size.

The real work begins when we decide there is a shortage of huge pages.  What
happens next depends on whether the pages are for a private or shared mapping.
Private mappings are straightforward.  At fault time, if alloc_huge_page()
fails, we allocate a page from the buddy allocator and increment the source
node's surplus_huge_pages counter.  When free_huge_page() is called for a page
on a node with a surplus, the page is freed directly to the buddy allocator
instead of the hugetlb pool.

Because shared mappings require all of the pages to be reserved up front, some
additional work must be done at mmap() to support them.  We determine the
reservation shortage and allocate the required number of pages all at once.
These pages are then added to the hugetlb pool and marked reserved.  Where that
is not possible the mmap() will fail.  As with private mappings, the
appropriate surplus counters are updated.  Since reserved huge pages won't
necessarily be used by the process, we can't be sure that free_huge_page() will
always be called to return surplus pages to the buddy allocator.  To prevent
the huge page pool from bloating, we must free unused surplus pages when their
reservation has ended.

Controlling it
==============

With the entire patch series applied, pool resizing is off by default so unless
specific action is taken, the semantics are unchanged.

To take advantage of the flexibility afforded by this patch series one must
tolerate a change in semantics.  To control hugetlb pool growth, the following
techniques can be employed:

 * A sysctl tunable to enable/disable the feature entirely
 * The size= mount option for hugetlbfs filesystems to limit pool size

Changelog
=========

9/24/2007 - Version 4
 - Smart handling of surplus pages when explictly resizing the pool
 - Fixed some counting bugs found in patch review

9/17/2007 - Version 3
 - fixed gather_surplus_pages 'needed' check
 - Removed 'locked_vm' changes from this series since that is really a
   separate logical change

9/12/2007 - Version 2
 - Integrated the cpuset_mems_nr() best-effort reservation check
 - Surplus pool allocations respect cpuset and numa policy restrictions
 - Unused surplus pages that are part of a reservation are freed when the
   reservation is released

7/13/2007 - Initial Post

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[PATCH 1/4] hugetlb: Move update_and_free_page

[Adam Litke]

This patch simply moves update_and_free_page() so that it can be reused
later in this patch series.  The implementation is not changed.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Acked-by: Andy Whitcroft <apw@shadowen.org>
Acked-by: Dave McCracken <dave.mccracken@oracle.com>
---

 mm/hugetlb.c |   30 +++++++++++++++---------------
 1 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index de4cf45..eb5b9f4 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -90,6 +90,21 @@ static struct page *dequeue_huge_page(struct vm_area_struct *vma,
 	return page;
 }
 
+static void update_and_free_page(struct page *page)
+{
+	int i;
+	nr_huge_pages--;
+	nr_huge_pages_node[page_to_nid(page)]--;
+	for (i = 0; i < (HPAGE_SIZE / PAGE_SIZE); i++) {
+		page[i].flags &= ~(1 << PG_locked | 1 << PG_error | 1 << PG_referenced |
+				1 << PG_dirty | 1 << PG_active | 1 << PG_reserved |
+				1 << PG_private | 1<< PG_writeback);
+	}
+	set_compound_page_dtor(page, NULL);
+	set_page_refcounted(page);
+	__free_pages(page, HUGETLB_PAGE_ORDER);
+}
+
 static void free_huge_page(struct page *page)
 {
 	BUG_ON(page_count(page));
@@ -199,21 +214,6 @@ static unsigned int cpuset_mems_nr(unsigned int *array)
 }
 
 #ifdef CONFIG_SYSCTL
-static void update_and_free_page(struct page *page)
-{
-	int i;
-	nr_huge_pages--;
-	nr_huge_pages_node[page_to_nid(page)]--;
-	for (i = 0; i < (HPAGE_SIZE / PAGE_SIZE); i++) {
-		page[i].flags &= ~(1 << PG_locked | 1 << PG_error | 1 << PG_referenced |
-				1 << PG_dirty | 1 << PG_active | 1 << PG_reserved |
-				1 << PG_private | 1<< PG_writeback);
-	}
-	set_compound_page_dtor(page, NULL);
-	set_page_refcounted(page);
-	__free_pages(page, HUGETLB_PAGE_ORDER);
-}
-
 #ifdef CONFIG_HIGHMEM
 static void try_to_free_low(unsigned long count)
 {

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[PATCH 2/4] hugetlb: Try to grow hugetlb pool for MAP_PRIVATE mappings

[Adam Litke]

Because we overcommit hugepages for MAP_PRIVATE mappings, it is possible
that the hugetlb pool will be exhausted or completely reserved when a
hugepage is needed to satisfy a page fault.  Before killing the process in
this situation, try to allocate a hugepage directly from the buddy
allocator.

The explicitly configured pool size becomes a low watermark.  When
dynamically grown, the allocated huge pages are accounted as a surplus over
the watermark.  As huge pages are freed on a node, surplus pages are
released to the buddy allocator so that the pool will shrink back to the
watermark.

Surplus accounting also allows for friendlier explicit pool resizing.  When
shrinking a pool that is fully in-use, increase the surplus so pages will
be returned to the buddy allocator as soon as they are freed.  When growing
a pool that has a surplus, consume the surplus first and then allocate new
pages.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Andy Whitcroft <apw@shadowen.org>
Acked-by: Dave McCracken <dave.mccracken@oracle.com>
---

 mm/hugetlb.c |  134 ++++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 120 insertions(+), 14 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index eb5b9f4..fe93cac 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -23,10 +23,12 @@
 
 const unsigned long hugetlb_zero = 0, hugetlb_infinity = ~0UL;
 static unsigned long nr_huge_pages, free_huge_pages, resv_huge_pages;
+static unsigned long surplus_huge_pages;
 unsigned long max_huge_pages;
 static struct list_head hugepage_freelists[MAX_NUMNODES];
 static unsigned int nr_huge_pages_node[MAX_NUMNODES];
 static unsigned int free_huge_pages_node[MAX_NUMNODES];
+static unsigned int surplus_huge_pages_node[MAX_NUMNODES];
 static gfp_t htlb_alloc_mask = GFP_HIGHUSER;
 unsigned long hugepages_treat_as_movable;
 
@@ -107,15 +109,52 @@ static void update_and_free_page(struct page *page)
 
 static void free_huge_page(struct page *page)
 {
-	BUG_ON(page_count(page));
+	int nid = page_to_nid(page);
 
+	BUG_ON(page_count(page));
 	INIT_LIST_HEAD(&page->lru);
 
 	spin_lock(&hugetlb_lock);
-	enqueue_huge_page(page);
+	if (surplus_huge_pages_node[nid]) {
+		update_and_free_page(page);
+		surplus_huge_pages--;
+		surplus_huge_pages_node[nid]--;
+	} else {
+		enqueue_huge_page(page);
+	}
 	spin_unlock(&hugetlb_lock);
 }
 
+/*
+ * Increment or decrement surplus_huge_pages.  Keep node-specific counters
+ * balanced by operating on them in a round-robin fashion.
+ */
+static int adjust_pool_surplus(int delta)
+{
+	static int prev_nid;
+	int nid, ret = 0;
+
+	nid = prev_nid;
+	do {
+		nid = next_node(nid, node_online_map);
+		if (nid == MAX_NUMNODES)
+			nid = first_node(node_online_map);
+
+		if (delta < 0 && !surplus_huge_pages_node[nid])
+			continue;
+		if (delta > 0 && !nr_huge_pages_node[nid])
+			continue;
+
+		surplus_huge_pages += delta;
+		surplus_huge_pages_node[nid] += delta;
+		ret = 1;
+		break;
+	} while (nid != prev_nid);
+
+	prev_nid = nid;
+	return ret;
+}
+
 static int alloc_fresh_huge_page(void)
 {
 	static int prev_nid;
@@ -148,10 +187,30 @@ static int alloc_fresh_huge_page(void)
 	return 0;
 }
 
+static struct page *alloc_buddy_huge_page(struct vm_area_struct *vma,
+						unsigned long address)
+{
+	struct page *page;
+
+	page = alloc_pages(htlb_alloc_mask|__GFP_COMP|__GFP_NOWARN,
+					HUGETLB_PAGE_ORDER);
+	if (page) {
+		set_compound_page_dtor(page, free_huge_page);
+		spin_lock(&hugetlb_lock);
+		nr_huge_pages++;
+		nr_huge_pages_node[page_to_nid(page)]++;
+		surplus_huge_pages++;
+		surplus_huge_pages_node[page_to_nid(page)]++;
+		spin_unlock(&hugetlb_lock);
+	}
+
+	return page;
+}
+
 static struct page *alloc_huge_page(struct vm_area_struct *vma,
 				    unsigned long addr)
 {
-	struct page *page;
+	struct page *page = NULL;
 
 	spin_lock(&hugetlb_lock);
 	if (vma->vm_flags & VM_MAYSHARE)
@@ -171,7 +230,16 @@ fail:
 	if (vma->vm_flags & VM_MAYSHARE)
 		resv_huge_pages++;
 	spin_unlock(&hugetlb_lock);
-	return NULL;
+
+	/*
+	 * Private mappings do not use reserved huge pages so the allocation
+	 * may have failed due to an undersized hugetlb pool.  Try to grab a
+	 * surplus huge page from the buddy allocator.
+	 */
+	if (!(vma->vm_flags & VM_MAYSHARE))
+		page = alloc_buddy_huge_page(vma, addr);
+
+	return page;
 }
 
 static int __init hugetlb_init(void)
@@ -239,26 +307,62 @@ static inline void try_to_free_low(unsigned long count)
 }
 #endif
 
+#define persistent_huge_pages (nr_huge_pages - surplus_huge_pages)
 static unsigned long set_max_huge_pages(unsigned long count)
 {
-	while (count > nr_huge_pages) {
-		if (!alloc_fresh_huge_page())
-			return nr_huge_pages;
-	}
-	if (count >= nr_huge_pages)
-		return nr_huge_pages;
+	unsigned long min_count, ret;
 
+	/*
+	 * Increase the pool size
+	 * First take pages out of surplus state.  Then make up the
+	 * remaining difference by allocating fresh huge pages.
+	 */
 	spin_lock(&hugetlb_lock);
-	count = max(count, resv_huge_pages);
-	try_to_free_low(count);
-	while (count < nr_huge_pages) {
+	while (surplus_huge_pages && count > persistent_huge_pages) {
+		if (!adjust_pool_surplus(-1))
+			break;
+	}
+
+	while (count > persistent_huge_pages) {
+		int ret;
+		/*
+		 * If this allocation races such that we no longer need the
+		 * page, free_huge_page will handle it by freeing the page
+		 * and reducing the surplus.
+		 */
+		spin_unlock(&hugetlb_lock);
+		ret = alloc_fresh_huge_page();
+		spin_lock(&hugetlb_lock);
+		if (!ret)
+			goto out;
+
+	}
+	if (count >= persistent_huge_pages)
+		goto out;
+
+	/*
+	 * Decrease the pool size
+	 * First return free pages to the buddy allocator (being careful
+	 * to keep enough around to satisfy reservations).  Then place
+	 * pages into surplus state as needed so the pool will shrink
+	 * to the desired size as pages become free.
+	 */
+	min_count = max(count, resv_huge_pages);
+	try_to_free_low(min_count);
+	while (min_count < persistent_huge_pages) {
 		struct page *page = dequeue_huge_page(NULL, 0);
 		if (!page)
 			break;
 		update_and_free_page(page);
 	}
+	while (count < persistent_huge_pages) {
+		if (!adjust_pool_surplus(1))
+			break;
+	}
+out:
+	ret = persistent_huge_pages;
 	spin_unlock(&hugetlb_lock);
-	return nr_huge_pages;
+	return ret;
 }
 
 int hugetlb_sysctl_handler(struct ctl_table *table, int write,
@@ -290,10 +394,12 @@ int hugetlb_report_meminfo(char *buf)
 			"HugePages_Total: %5lu\n"
 			"HugePages_Free:  %5lu\n"
 			"HugePages_Rsvd:  %5lu\n"
+			"HugePages_Surp:  %5lu\n"
 			"Hugepagesize:    %5lu kB\n",
 			nr_huge_pages,
 			free_huge_pages,
 			resv_huge_pages,
+			surplus_huge_pages,
 			HPAGE_SIZE/1024);
 }
 

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[PATCH 3/4] hugetlb: Try to grow hugetlb pool for MAP_SHARED mappings

[Adam Litke]

Shared mappings require special handling because the huge pages needed to
fully populate the VMA must be reserved at mmap time.  If not enough pages
are available when making the reservation, allocate all of the shortfall at
once from the buddy allocator and add the pages directly to the hugetlb
pool.  If they cannot be allocated, then fail the mapping.  The page
surplus is accounted for in the same way as for private mappings; faulted
surplus pages will be freed at unmap time.  Reserved, surplus pages that
have not been used must be freed separately when their reservation has been
released.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Acked-by: Andy Whitcroft <apw@shadowen.org>
Acked-by: Dave McCracken <dave.mccracken@oracle.com>
---

 mm/hugetlb.c |  155 +++++++++++++++++++++++++++++++++++++++++++++++++---------
 1 files changed, 132 insertions(+), 23 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index fe93cac..ea77cb8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -86,6 +86,8 @@ static struct page *dequeue_huge_page(struct vm_area_struct *vma,
 			list_del(&page->lru);
 			free_huge_pages--;
 			free_huge_pages_node[nid]--;
+			if (vma && vma->vm_flags & VM_MAYSHARE)
+				resv_huge_pages--;
 			break;
 		}
 	}
@@ -207,15 +209,116 @@ static struct page *alloc_buddy_huge_page(struct vm_area_struct *vma,
 	return page;
 }
 
+/*
+ * Increase the hugetlb pool such that it can accomodate a reservation
+ * of size 'delta'.
+ */
+static int gather_surplus_pages(int delta)
+{
+	struct list_head surplus_list;
+	struct page *page, *tmp;
+	int ret, i;
+	int needed, allocated;
+
+	needed = (resv_huge_pages + delta) - free_huge_pages;
+	if (needed <= 0)
+		return 0;
+
+	allocated = 0;
+	INIT_LIST_HEAD(&surplus_list);
+
+	ret = -ENOMEM;
+retry:
+	spin_unlock(&hugetlb_lock);
+	for (i = 0; i < needed; i++) {
+		page = alloc_buddy_huge_page(NULL, 0);
+		if (!page) {
+			/*
+			 * We were not able to allocate enough pages to
+			 * satisfy the entire reservation so we free what
+			 * we've allocated so far.
+			 */
+			spin_lock(&hugetlb_lock);
+			needed = 0;
+			goto free;
+		}
+
+		list_add(&page->lru, &surplus_list);
+	}
+	allocated += needed;
+
+	/*
+	 * After retaking hugetlb_lock, we need to recalculate 'needed'
+	 * because either resv_huge_pages or free_huge_pages may have changed.
+	 */
+	spin_lock(&hugetlb_lock);
+	needed = (resv_huge_pages + delta) - (free_huge_pages + allocated);
+	if (needed > 0)
+		goto retry;
+
+	/*
+	 * The surplus_list now contains _at_least_ the number of extra pages
+	 * needed to accomodate the reservation.  Add the appropriate number
+	 * of pages to the hugetlb pool and free the extras back to the buddy
+	 * allocator.
+	 */
+	needed += allocated;
+	ret = 0;
+free:
+	list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
+		list_del(&page->lru);
+		if ((--needed) >= 0)
+			enqueue_huge_page(page);
+		else
+			update_and_free_page(page);
+	}
+
+	return ret;
+}
+
+/*
+ * When releasing a hugetlb pool reservation, any surplus pages that were
+ * allocated to satisfy the reservation must be explicitly freed if they were
+ * never used.
+ */
+void return_unused_surplus_pages(unsigned long unused_resv_pages)
+{
+	static int nid = -1;
+	struct page *page;
+	unsigned long nr_pages;
+
+	nr_pages = min(unused_resv_pages, surplus_huge_pages);
+
+	while (nr_pages) {
+		nid = next_node(nid, node_online_map);
+		if (nid == MAX_NUMNODES)
+			nid = first_node(node_online_map);
+
+		if (!surplus_huge_pages_node[nid])
+			continue;
+
+		if (!list_empty(&hugepage_freelists[nid])) {
+			page = list_entry(hugepage_freelists[nid].next,
+					  struct page, lru);
+			list_del(&page->lru);
+			update_and_free_page(page);
+			free_huge_pages--;
+			free_huge_pages_node[nid]--;
+			surplus_huge_pages--;
+			surplus_huge_pages_node[nid]--;
+			nr_pages--;
+		}
+	}
+}
+
 static struct page *alloc_huge_page(struct vm_area_struct *vma,
 				    unsigned long addr)
 {
 	struct page *page = NULL;
+	int use_reserved_page = vma->vm_flags & VM_MAYSHARE;
 
 	spin_lock(&hugetlb_lock);
-	if (vma->vm_flags & VM_MAYSHARE)
-		resv_huge_pages--;
-	else if (free_huge_pages <= resv_huge_pages)
+	if (!use_reserved_page && (free_huge_pages <= resv_huge_pages))
 		goto fail;
 
 	page = dequeue_huge_page(vma, addr);
@@ -227,8 +330,6 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
 	return page;
 
 fail:
-	if (vma->vm_flags & VM_MAYSHARE)
-		resv_huge_pages++;
 	spin_unlock(&hugetlb_lock);
 
 	/*
@@ -236,7 +337,7 @@ fail:
 	 * may have failed due to an undersized hugetlb pool.  Try to grab a
 	 * surplus huge page from the buddy allocator.
 	 */
-	if (!(vma->vm_flags & VM_MAYSHARE))
+	if (!use_reserved_page)
 		page = alloc_buddy_huge_page(vma, addr);
 
 	return page;
@@ -947,21 +1048,6 @@ static int hugetlb_acct_memory(long delta)
 	int ret = -ENOMEM;
 
 	spin_lock(&hugetlb_lock);
-	if ((delta + resv_huge_pages) <= free_huge_pages) {
-		resv_huge_pages += delta;
-		ret = 0;
-	}
-	spin_unlock(&hugetlb_lock);
-	return ret;
-}
-
-int hugetlb_reserve_pages(struct inode *inode, long from, long to)
-{
-	long ret, chg;
-
-	chg = region_chg(&inode->i_mapping->private_list, from, to);
-	if (chg < 0)
-		return chg;
 	/*
 	 * When cpuset is configured, it breaks the strict hugetlb page
 	 * reservation as the accounting is done on a global variable. Such
@@ -979,8 +1065,31 @@ int hugetlb_reserve_pages(struct inode *inode, long from, long to)
 	 * a best attempt and hopefully to minimize the impact of changing
 	 * semantics that cpuset has.
 	 */
-	if (chg > cpuset_mems_nr(free_huge_pages_node))
-		return -ENOMEM;
+	if (delta > 0) {
+		if (gather_surplus_pages(delta) < 0)
+			goto out;
+
+		if (delta > cpuset_mems_nr(free_huge_pages_node))
+			goto out;
+	}
+
+	ret = 0;
+	resv_huge_pages += delta;
+	if (delta < 0)
+		return_unused_surplus_pages((unsigned long) -delta);
+
+out:
+	spin_unlock(&hugetlb_lock);
+	return ret;
+}
+
+int hugetlb_reserve_pages(struct inode *inode, long from, long to)
+{
+	long ret, chg;
+
+	chg = region_chg(&inode->i_mapping->private_list, from, to);
+	if (chg < 0)
+		return chg;
 
 	ret = hugetlb_acct_memory(chg);
 	if (ret < 0)

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[PATCH 4/4] hugetlb: Add hugetlb_dynamic_pool sysctl

[Adam Litke]

The maximum size of the huge page pool can be controlled using the overall
size of the hugetlb filesystem (via its 'size' mount option).  However in
the common case the this will not be set as the pool is traditionally fixed
in size at boot time.  In order to maintain the expected semantics, we need
to prevent the pool expanding by default.

This patch introduces a new sysctl controlling dynamic pool resizing.  When
this is enabled the pool will expand beyond its base size up to the size of
the hugetlb filesystem.  It is disabled by default.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Acked-by: Andy Whitcroft <apw@shadowen.org>
Acked-by: Dave McCracken <dave.mccracken@oracle.com>
---

 include/linux/hugetlb.h |    1 +
 kernel/sysctl.c         |    8 ++++++++
 mm/hugetlb.c            |    5 +++++
 3 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 3a19b03..ea0f50b 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -33,6 +33,7 @@ void hugetlb_unreserve_pages(struct inode *inode, long offset, long freed);
 
 extern unsigned long max_huge_pages;
 extern unsigned long hugepages_treat_as_movable;
+extern int hugetlb_dynamic_pool;
 extern const unsigned long hugetlb_zero, hugetlb_infinity;
 extern int sysctl_hugetlb_shm_group;
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 6ace893..45e8045 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -889,6 +889,14 @@ static ctl_table vm_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &hugetlb_treat_movable_handler,
 	},
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "hugetlb_dynamic_pool",
+		.data		= &hugetlb_dynamic_pool,
+		.maxlen		= sizeof(hugetlb_dynamic_pool),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 #endif
 	{
 		.ctl_name	= VM_LOWMEM_RESERVE_RATIO,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ea77cb8..8e9ea65 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -31,6 +31,7 @@ static unsigned int free_huge_pages_node[MAX_NUMNODES];
 static unsigned int surplus_huge_pages_node[MAX_NUMNODES];
 static gfp_t htlb_alloc_mask = GFP_HIGHUSER;
 unsigned long hugepages_treat_as_movable;
+int hugetlb_dynamic_pool;
 
 /*
  * Protects updates to hugepage_freelists, nr_huge_pages, and free_huge_pages
@@ -194,6 +195,10 @@ static struct page *alloc_buddy_huge_page(struct vm_area_struct *vma,
 {
 	struct page *page;
 
+	/* Check if the dynamic pool is enabled */
+	if (!hugetlb_dynamic_pool)
+		return NULL;
+
 	page = alloc_pages(htlb_alloc_mask|__GFP_COMP|__GFP_NOWARN,
 					HUGETLB_PAGE_ORDER);
 	if (page) {

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Re: [PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[Balbir Singh]

Adam Litke wrote:
> How it works
> ============
> 
> Upon depletion of the hugetlb pool, rather than reporting an error immediately,
> first try and allocate the needed huge pages directly from the buddy allocator.
> Care must be taken to avoid unbounded growth of the hugetlb pool, so the
> hugetlb filesystem quota is used to limit overall pool size.
> 

If I understand hugetlb correctly, there is no accounting of hugepages
to the RSS of any process. Since the pool will no longer be static,
should we also consider changes to the accounting of hugepages?



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	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL

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Re: [PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[Adam Litke]

On Tue, 2007-09-25 at 16:52 +0530, Balbir Singh wrote:
> Adam Litke wrote:
> > How it works
> > ============
> > 
> > Upon depletion of the hugetlb pool, rather than reporting an error immediately,
> > first try and allocate the needed huge pages directly from the buddy allocator.
> > Care must be taken to avoid unbounded growth of the hugetlb pool, so the
> > hugetlb filesystem quota is used to limit overall pool size.
> > 
> 
> If I understand hugetlb correctly, there is no accounting of hugepages
> to the RSS of any process. Since the pool will no longer be static,
> should we also consider changes to the accounting of hugepages?

You're right: there is no accounting of huge pages against a process.
This is also the case for the statically allocated pool so this
particular issue exists unconditionally.  There are several things
missing: RSS accounting, counting huge pages towards locked_vm limits,
etc...  The plan is to address these separately and to fix them all at
once.

In the absence of traditional per-process huge page accounting, the
kernel has provided an alternate means for restricting a process' access
to the global hugetlb pool: filesystem permissions and quotas.  It's not
ideal, but with this patch series, the filesystem permissions and quotas
remain the effective mechanism for restricting pool growth and
consumption by processes.

-- 
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center

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Re: [PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[Balbir Singh]

Adam Litke wrote:
> On Tue, 2007-09-25 at 16:52 +0530, Balbir Singh wrote:
>> Adam Litke wrote:
>>> How it works
>>> ============
>>>
>>> Upon depletion of the hugetlb pool, rather than reporting an error immediately,
>>> first try and allocate the needed huge pages directly from the buddy allocator.
>>> Care must be taken to avoid unbounded growth of the hugetlb pool, so the
>>> hugetlb filesystem quota is used to limit overall pool size.
>>>
>> If I understand hugetlb correctly, there is no accounting of hugepages
>> to the RSS of any process. Since the pool will no longer be static,
>> should we also consider changes to the accounting of hugepages?
> 
> You're right: there is no accounting of huge pages against a process.
> This is also the case for the statically allocated pool so this
> particular issue exists unconditionally.  There are several things
> missing: RSS accounting, counting huge pages towards locked_vm limits,
> etc...  The plan is to address these separately and to fix them all at
> once.
> 

I am interested in the accounting and control of hugepages as an
extension to the current memory controller, we can of-course do this
incrementally.

> In the absence of traditional per-process huge page accounting, the
> kernel has provided an alternate means for restricting a process' access
> to the global hugetlb pool: filesystem permissions and quotas.  It's not
> ideal, but with this patch series, the filesystem permissions and quotas
> remain the effective mechanism for restricting pool growth and
> consumption by processes.
> 

OK, thats what I thought.

Thanks for sharing your plans


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	Balbir Singh
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	IBM, ISTL

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[PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[Adam Litke]

*** Series updated to remove locked_vm accounting
The upper bound on pool growth is governed by per-filesystem quotas which
maintains the global nature of huge page usage limits.  Per process accounting
of hugepages as locked memory has been pulled out of this patch series as it is
logically separate, and will be pushed separately.
***

In most real-world scenarios, configuring the size of the hugetlb pool
correctly is a difficult task.  If too few pages are allocated to the pool,
applications using MAP_SHARED may fail to mmap() a hugepage region and
applications using MAP_PRIVATE may receive SIGBUS.  Isolating too much memory
in the hugetlb pool means it is not available for other uses, especially those
programs not using huge pages.

The obvious answer is to let the hugetlb pool grow and shrink in response to
the runtime demand for huge pages.  The work Mel Gorman has been doing to
establish a memory zone for movable memory allocations makes dynamically
resizing the hugetlb pool reliable within the limits of that zone.  This patch
series implements dynamic pool resizing for private and shared mappings while
being careful to maintain existing semantics.  Please reply with your comments
and feedback; even just to say whether it would be a useful feature to you.
Thanks.

How it works
============

Upon depletion of the hugetlb pool, rather than reporting an error immediately,
first try and allocate the needed huge pages directly from the buddy allocator.
Care must be taken to avoid unbounded growth of the hugetlb pool, so the
hugetlb filesystem quota is used to limit overall pool size.

The real work begins when we decide there is a shortage of huge pages.  What
happens next depends on whether the pages are for a private or shared mapping.
Private mappings are straightforward.  At fault time, if alloc_huge_page()
fails, we allocate a page from the buddy allocator and increment the source
node's surplus_huge_pages counter.  When free_huge_page() is called for a page
on a node with a surplus, the page is freed directly to the buddy allocator
instead of the hugetlb pool.

Because shared mappings require all of the pages to be reserved up front, some
additional work must be done at mmap() to support them.  We determine the
reservation shortage and allocate the required number of pages all at once.
These pages are then added to the hugetlb pool and marked reserved.  Where that
is not possible the mmap() will fail.  As with private mappings, the
appropriate surplus counters are updated.  Since reserved huge pages won't
necessarily be used by the process, we can't be sure that free_huge_page() will
always be called to return surplus pages to the buddy allocator.  To prevent
the huge page pool from bloating, we must free unused surplus pages when their
reservation has ended.

Controlling it
==============

With the entire patch series applied, pool resizing is off by default so unless
specific action is taken, the semantics are unchanged.

To take advantage of the flexibility afforded by this patch series one must
tolerate a change in semantics.  To control hugetlb pool growth, the following
techniques can be employed:

 * A sysctl tunable to enable/disable the feature entirely
 * The size= mount option for hugetlbfs filesystems to limit pool size

Future Improvements
===================

I am aware of the following issues and plan to address then in the future as
separate changes since they are not critical and to keep this patch series as
small as possible.

 * Modifying the pool size (via sysctl) while there is a pool surplus could be
   made smarter.  Right now the surplus is ignored which can result in unneeded
   alloc_fresh_huge_page() calls.

Changelog
=========

9/17/2007 - Version 3
 - fixed gather_surplus_pages 'needed' check
 - Removed 'locked_vm' changes from this series since that is really a
   separate logical change

9/12/2007 - Version 2
 - Integrated the cpuset_mems_nr() best-effort reservation check
 - Surplus pool allocations respect cpuset and numa policy restrictions
 - Unused surplus pages that are part of a reservation are freed when the
   reservation is released

7/13/2007 - Initial Post

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Re: [PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[Dave McCracken]

On Monday 17 September 2007, Adam Litke wrote:
> In most real-world scenarios, configuring the size of the hugetlb pool
> correctly is a difficult task. A If too few pages are allocated to the pool,
> applications using MAP_SHARED may fail to mmap() a hugepage region and
> applications using MAP_PRIVATE may receive SIGBUS. A Isolating too much
> memory in the hugetlb pool means it is not available for other uses,
> especially those programs not using huge pages.
>
> The obvious answer is to let the hugetlb pool grow and shrink in response
> to the runtime demand for huge pages. A The work Mel Gorman has been doing
> to establish a memory zone for movable memory allocations makes dynamically
> resizing the hugetlb pool reliable within the limits of that zone. A This
> patch series implements dynamic pool resizing for private and shared
> mappings while being careful to maintain existing semantics. A Please reply
> with your comments and feedback; even just to say whether it would be a
> useful feature to you. Thanks.

Now that we have Mel's mobility patches to make it feasible to dynamically 
allocate huge pages, I'd say it's definitely time to get this patch in.  
Users will really appreciate not having to preallocate huge pages for all 
possible workloads.

Dave

Acked-by: Dave McCracken <dave.mccracken@oracle.com>

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Re: [PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[Andrew Hastings]

Adam Litke wrote:
> The obvious answer is to let the hugetlb pool grow and shrink in response to
> the runtime demand for huge pages.  The work Mel Gorman has been doing to
> establish a memory zone for movable memory allocations makes dynamically
> resizing the hugetlb pool reliable within the limits of that zone.  This patch
> series implements dynamic pool resizing for private and shared mappings while
> being careful to maintain existing semantics.  Please reply with your comments
> and feedback; even just to say whether it would be a useful feature to you.

Thanks, this will be extremely useful for our customers' workloads.

-Andrew Hastings
  Cray Inc.

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Re: [PATCH 0/4] [hugetlb] Dynamic huge page pool resizing

[Avi Kivity]

Adam Litke wrote:
> *** Series updated to remove locked_vm accounting
> The upper bound on pool growth is governed by per-filesystem quotas which
> maintains the global nature of huge page usage limits.  Per process accounting
> of hugepages as locked memory has been pulled out of this patch series as it is
> logically separate, and will be pushed separately.
> ***
>
> In most real-world scenarios, configuring the size of the hugetlb pool
> correctly is a difficult task.  If too few pages are allocated to the pool,
> applications using MAP_SHARED may fail to mmap() a hugepage region and
> applications using MAP_PRIVATE may receive SIGBUS.  Isolating too much memory
> in the hugetlb pool means it is not available for other uses, especially those
> programs not using huge pages.
>
> The obvious answer is to let the hugetlb pool grow and shrink in response to
> the runtime demand for huge pages.  The work Mel Gorman has been doing to
> establish a memory zone for movable memory allocations makes dynamically
> resizing the hugetlb pool reliable within the limits of that zone.  This patch
> series implements dynamic pool resizing for private and shared mappings while
> being careful to maintain existing semantics.  Please reply with your comments
> and feedback; even just to say whether it would be a useful feature to you.
> Thanks.
>
>   

kvm with newer hardware (that supports nested paging) stands to benefit 
greatly from this. backing guest memory with huge pages significantly 
decreases tlb miss costs, and allows using huge tlb entries for guest 
kernel mappings.


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