diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index b351952899..1a1c2fc 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -56,48 +56,10 @@
 	pagevec_reinit(pvec);
 }
 
-/*
- * huge_pages_needed tries to determine the number of new huge pages that
- * will be required to fully populate this VMA.  This will be equal to
- * the size of the VMA in huge pages minus the number of huge pages
- * (covered by this VMA) that are found in the page cache.
- *
- * Result is in bytes to be compatible with is_hugepage_mem_enough()
- */
-static unsigned long
-huge_pages_needed(struct address_space *mapping, struct vm_area_struct *vma)
-{
-	int i;
-	struct pagevec pvec;
-	unsigned long start = vma->vm_start;
-	unsigned long end = vma->vm_end;
-	unsigned long hugepages = (end - start) >> HPAGE_SHIFT;
-	pgoff_t next = vma->vm_pgoff >> (HPAGE_SHIFT - PAGE_SHIFT);
-	pgoff_t endpg = next + hugepages;
-
-	pagevec_init(&pvec, 0);
-	while (next < endpg) {
-		if (!pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE))
-			break;
-		for (i = 0; i < pagevec_count(&pvec); i++) {
-			struct page *page = pvec.pages[i];
-			if (page->index > next)
-				next = page->index;
-			if (page->index >= endpg)
-				break;
-			next++;
-			hugepages--;
-		}
-		huge_pagevec_release(&pvec);
-	}
-	return hugepages << HPAGE_SHIFT;
-}
-
 static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct inode *inode = file->f_dentry->d_inode;
-	struct address_space *mapping = inode->i_mapping;
-	unsigned long bytes;
+	struct hugetlbfs_inode_info *info = HUGETLBFS_I(inode);
 	loff_t len, vma_len;
 	int ret;
 
@@ -113,10 +75,6 @@
 	if (vma->vm_end - vma->vm_start < HPAGE_SIZE)
 		return -EINVAL;
 
-	bytes = huge_pages_needed(mapping, vma);
-	if (!is_hugepage_mem_enough(bytes))
-		return -ENOMEM;
-
 	vma_len = (loff_t)(vma->vm_end - vma->vm_start);
 
 	mutex_lock(&inode->i_mutex);
@@ -129,6 +87,10 @@
 	if (!(vma->vm_flags & VM_WRITE) && len > inode->i_size)
 		goto out;
 
+	if (vma->vm_flags & VM_MAYSHARE)
+		if (hugetlb_extend_reservation(info, len >> HPAGE_SHIFT) != 0)
+			goto out;
+
 	ret = 0;
 	hugetlb_prefault_arch_hook(vma->vm_mm);
 	if (inode->i_size < len)
@@ -227,13 +189,18 @@
 	put_page(page);
 }
 
-static void truncate_hugepages(struct address_space *mapping, loff_t lstart)
+static void truncate_hugepages(struct inode *inode, loff_t lstart)
 {
+	struct address_space *mapping = &inode->i_data;
 	const pgoff_t start = lstart >> HPAGE_SHIFT;
 	struct pagevec pvec;
 	pgoff_t next;
 	int i;
 
+	hugetlb_truncate_reservation(HUGETLBFS_I(inode),
+				     lstart >> HPAGE_SHIFT);
+	if (!mapping->nrpages)
+		return;
 	pagevec_init(&pvec, 0);
 	next = start;
 	while (1) {
@@ -262,8 +229,7 @@
 
 static void hugetlbfs_delete_inode(struct inode *inode)
 {
-	if (inode->i_data.nrpages)
-		truncate_hugepages(&inode->i_data, 0);
+	truncate_hugepages(inode, 0);
 	clear_inode(inode);
 }
 
@@ -296,8 +262,7 @@
 	inode->i_state |= I_FREEING;
 	inodes_stat.nr_inodes--;
 	spin_unlock(&inode_lock);
-	if (inode->i_data.nrpages)
-		truncate_hugepages(&inode->i_data, 0);
+	truncate_hugepages(inode, 0);
 	clear_inode(inode);
 	destroy_inode(inode);
 }
@@ -356,7 +321,7 @@
 	if (!prio_tree_empty(&mapping->i_mmap))
 		hugetlb_vmtruncate_list(&mapping->i_mmap, pgoff);
 	spin_unlock(&mapping->i_mmap_lock);
-	truncate_hugepages(mapping, offset);
+	truncate_hugepages(inode, offset);
 	return 0;
 }
 
@@ -573,6 +538,7 @@
 		hugetlbfs_inc_free_inodes(sbinfo);
 		return NULL;
 	}
+	p->prereserved_hpages = 0;
 	return &p->vfs_inode;
 }
 
@@ -805,9 +771,6 @@
 	if (!can_do_hugetlb_shm())
 		return ERR_PTR(-EPERM);
 
-	if (!is_hugepage_mem_enough(size))
-		return ERR_PTR(-ENOMEM);
-
 	if (!user_shm_lock(size, current->user))
 		return ERR_PTR(-ENOMEM);
 
@@ -831,6 +794,11 @@
 	if (!inode)
 		goto out_file;
 
+	error = -ENOMEM;
+	if (hugetlb_extend_reservation(HUGETLBFS_I(inode),
+				       size >> HPAGE_SHIFT) != 0)
+		goto out_inode;
+
 	d_instantiate(dentry, inode);
 	inode->i_size = size;
 	inode->i_nlink = 0;
@@ -841,6 +809,8 @@
 	file->f_mode = FMODE_WRITE | FMODE_READ;
 	return file;
 
+out_inode:
+	iput(inode);
 out_file:
 	put_filp(file);
 out_dentry:
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index fa83836..cafe73e 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -20,7 +20,6 @@
 int hugetlb_prefault(struct address_space *, struct vm_area_struct *);
 int hugetlb_report_meminfo(char *);
 int hugetlb_report_node_meminfo(int, char *);
-int is_hugepage_mem_enough(size_t);
 unsigned long hugetlb_total_pages(void);
 struct page *alloc_huge_page(struct vm_area_struct *, unsigned long);
 void free_huge_page(struct page *);
@@ -89,7 +88,6 @@
 #define copy_hugetlb_page_range(src, dst, vma)	({ BUG(); 0; })
 #define hugetlb_prefault(mapping, vma)		({ BUG(); 0; })
 #define unmap_hugepage_range(vma, start, end)	BUG()
-#define is_hugepage_mem_enough(size)		0
 #define hugetlb_report_meminfo(buf)		0
 #define hugetlb_report_node_meminfo(n, buf)	0
 #define follow_huge_pmd(mm, addr, pmd, write)	NULL
@@ -132,6 +130,8 @@
 
 struct hugetlbfs_inode_info {
 	struct shared_policy policy;
+	/* Protected by the (global) hugetlb_lock */
+	unsigned long prereserved_hpages;
 	struct inode vfs_inode;
 };
 
@@ -148,6 +148,10 @@
 extern struct file_operations hugetlbfs_file_operations;
 extern struct vm_operations_struct hugetlb_vm_ops;
 struct file *hugetlb_zero_setup(size_t);
+int hugetlb_extend_reservation(struct hugetlbfs_inode_info *info,
+			       unsigned long atleast_hpages);
+void hugetlb_truncate_reservation(struct hugetlbfs_inode_info *info,
+				  unsigned long atmost_hpages);
 int hugetlb_get_quota(struct address_space *mapping);
 void hugetlb_put_quota(struct address_space *mapping);
 
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d5987a8..27fad5d 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -22,7 +22,7 @@
 #include "internal.h"
 
 const unsigned long hugetlb_zero = 0, hugetlb_infinity = ~0UL;
-static unsigned long nr_huge_pages, free_huge_pages;
+static unsigned long nr_huge_pages, free_huge_pages, reserved_huge_pages;
 unsigned long max_huge_pages;
 static struct list_head hugepage_freelists[MAX_NUMNODES];
 static unsigned int nr_huge_pages_node[MAX_NUMNODES];
@@ -120,17 +120,136 @@
 
 struct page *alloc_huge_page(struct vm_area_struct *vma, unsigned long addr)
 {
+	struct inode *inode = vma->vm_file->f_dentry->d_inode;
 	struct page *page;
+	int use_reserve = 0;
+	unsigned long idx;
 
 	spin_lock(&hugetlb_lock);
-	page = dequeue_huge_page(vma, addr);
-	if (!page) {
-		spin_unlock(&hugetlb_lock);
-		return NULL;
+
+	if (vma->vm_flags & VM_MAYSHARE) {
+
+		/* idx = radix tree index, i.e. offset into file in
+		 * HPAGE_SIZE units */
+		idx = ((addr - vma->vm_start) >> HPAGE_SHIFT)
+			+ (vma->vm_pgoff >> (HPAGE_SHIFT - PAGE_SHIFT));
+
+		/* The hugetlbfs specific inode info stores the number
+		 * of "guaranteed available" (huge) pages.  That is,
+		 * the first 'prereserved_hpages' pages of the inode
+		 * are either already instantiated, or have been
+		 * pre-reserved (by hugetlb_reserve_for_inode()). Here
+		 * we're in the process of instantiating the page, so
+		 * we use this to determine whether to draw from the
+		 * pre-reserved pool or the truly free pool. */
+		if (idx < HUGETLBFS_I(inode)->prereserved_hpages)
+			use_reserve = 1;
 	}
+
+	if (!use_reserve) {
+		if (free_huge_pages <= reserved_huge_pages)
+			goto fail;
+	} else {
+		BUG_ON(reserved_huge_pages == 0);
+		reserved_huge_pages--;
+	}
+
+	page = dequeue_huge_page(vma, addr);
+	if (!page)
+		goto fail;
+
 	spin_unlock(&hugetlb_lock);
 	set_page_refcounted(page);
 	return page;
+
+ fail:
+	WARN_ON(use_reserve); /* reserved allocations shouldn't fail */
+	spin_unlock(&hugetlb_lock);
+	return NULL;
+}
+
+/* hugetlb_extend_reservation()
+ *
+ * Ensure that at least 'atleast' hugepages are, and will remain,
+ * available to instantiate the first 'atleast' pages of the given
+ * inode.  If the inode doesn't already have this many pages reserved
+ * or instantiated, set aside some hugepages in the reserved pool to
+ * satisfy later faults (or fail now if there aren't enough, rather
+ * than getting the SIGBUS later).
+ */
+int hugetlb_extend_reservation(struct hugetlbfs_inode_info *info,
+			       unsigned long atleast)
+{
+	struct inode *inode = &info->vfs_inode;
+	unsigned long change_in_reserve = 0;
+	int ret = 0;
+
+	spin_lock(&hugetlb_lock);
+	read_lock_irq(&inode->i_mapping->tree_lock);
+
+	if (info->prereserved_hpages >= atleast)
+		goto out;
+
+	/* Because we always call this on shared mappings, none of the
+	 * pages beyond info->prereserved_hpages can have been
+	 * instantiated, so we need to reserve all of them now. */
+	change_in_reserve = atleast - info->prereserved_hpages;
+
+	if ((reserved_huge_pages + change_in_reserve) > free_huge_pages) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	reserved_huge_pages += change_in_reserve;
+	info->prereserved_hpages = atleast;
+
+ out:
+	read_unlock_irq(&inode->i_mapping->tree_lock);
+	spin_unlock(&hugetlb_lock);
+
+	return ret;
+}
+
+/* hugetlb_truncate_reservation()
+ *
+ * This returns pages reserved for the given inode to the general free
+ * hugepage pool.  If the inode has any pages prereserved, but not
+ * instantiated, beyond offset (atmost << HPAGE_SIZE), then release
+ * them.
+ */
+void hugetlb_truncate_reservation(struct hugetlbfs_inode_info *info,
+				  unsigned long atmost)
+{
+	struct inode *inode = &info->vfs_inode;
+	struct address_space *mapping = inode->i_mapping;
+	unsigned long idx;
+	unsigned long change_in_reserve = 0;
+	struct page *page;
+
+	spin_lock(&hugetlb_lock);
+	read_lock_irq(&inode->i_mapping->tree_lock);
+
+	if (info->prereserved_hpages <= atmost)
+		goto out;
+
+	/* Count pages which were reserved, but not instantiated, and
+	 * which we can now release. */
+	for (idx = atmost; idx < info->prereserved_hpages; idx++) {
+		page = radix_tree_lookup(&mapping->page_tree, idx);
+		if (!page)
+			/* Pages which are already instantiated can't
+			 * be unreserved (and in fact have already
+			 * been removed from the reserved pool) */
+			change_in_reserve++;
+	}
+
+	BUG_ON(reserved_huge_pages < change_in_reserve);
+	reserved_huge_pages -= change_in_reserve;
+	info->prereserved_hpages = atmost;
+
+ out:
+	read_unlock_irq(&inode->i_mapping->tree_lock);
+	spin_unlock(&hugetlb_lock);
 }
 
 static int __init hugetlb_init(void)
@@ -238,9 +357,11 @@
 	return sprintf(buf,
 			"HugePages_Total: %5lu\n"
 			"HugePages_Free:  %5lu\n"
+		        "HugePages_Rsvd:  %5lu\n"
 			"Hugepagesize:    %5lu kB\n",
 			nr_huge_pages,
 			free_huge_pages,
+		        reserved_huge_pages,
 			HPAGE_SIZE/1024);
 }
 
@@ -253,11 +374,6 @@
 		nid, free_huge_pages_node[nid]);
 }
 
-int is_hugepage_mem_enough(size_t size)
-{
-	return (size + ~HPAGE_MASK)/HPAGE_SIZE <= free_huge_pages;
-}
-
 /* Return the number pages of memory we physically have, in PAGE_SIZE units. */
 unsigned long hugetlb_total_pages(void)
 {