[PATCH] hugepage: Strict page reservation for hugepage inodes

These days, hugepages are demand-allocated at first fault time.  There's a
somewhat dubious (and racy) heuristic when making a new mmap() to check if
there are enough available hugepages to fully satisfy that mapping.

A particularly obvious case where the heuristic breaks down is where a
process maps its hugepages not as a single chunk, but as a bunch of
individually mmap()ed (or shmat()ed) blocks without touching and
instantiating the pages in between allocations.  In this case the size of
each block is compared against the total number of available hugepages.
It's thus easy for the process to become overcommitted, because each block
mapping will succeed, although the total number of hugepages required by
all blocks exceeds the number available.  In particular, this defeats such
a program which will detect a mapping failure and adjust its hugepage usage
downward accordingly.

The patch below addresses this problem, by strictly reserving a number of
physical hugepages for hugepage inodes which have been mapped, but not
instatiated.  MAP_SHARED mappings are thus "safe" - they will fail on
mmap(), not later with an OOM SIGKILL.  MAP_PRIVATE mappings can still
trigger an OOM.  (Actually SHARED mappings can technically still OOM, but
only if the sysadmin explicitly reduces the hugepage pool between mapping
and instantiation)

This patch appears to address the problem at hand - it allows DB2 to start
correctly, for instance, which previously suffered the failure described
above.

This patch causes no regressions on the libhugetblfs testsuite, and makes a
test (designed to catch this problem) pass which previously failed (ppc64,
POWER5).

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Cc: William Lee Irwin III <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 126 insertions, 10 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d5987a87bbe5..27fad5d9bcf6 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 @@ void free_huge_page(struct page *page)
 
 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 @@ int hugetlb_report_meminfo(char *buf)
         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 @@ int hugetlb_report_node_meminfo(int nid, char *buf)
                 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)
 {