From: Larry Woodman <lwoodman@redhat.com>
Date: Thu, 17 Jan 2008 10:20:26 -0500
Subject: [mm] hugepages: leak due to pagetable page sharing
Message-id: 478F723A.2010609@redhat.com
O-Subject: [RHEL5-U2 patch] fix hugepages leak due to pagetable page sharing.
Bugzilla: 428612

The shared page table patch for hugetlb memory on x86 and x86_64
is causing a leak.  When a user of hugepages exits using this code
the system leaks some of the hugepages.

-------------------------------------------------------
Part of /proc/meminfo just before database startup:
HugePages_Total:  5500
HugePages_Free:   5500
HugePages_Rsvd:      0
Hugepagesize:     2048 kB

Just before shutdown:
HugePages_Total:  5500
HugePages_Free:   4475
HugePages_Rsvd:      0
Hugepagesize:     2048 kB

After shutdown:
HugePages_Total:  5500 HugePages_Free:   4988 HugePages_Rsvd:      0
Hugepagesize:     2048 kB
----------------------------------------------------------

The problem occurs durring a fork, in copy_hugetlb_page_range(). It
locates the dst_pte using
huge_pte_alloc().  Since huge_pte_alloc() calls huge_pmd_share() it will
share the pmd page
if can, yet the main loop in copy_hugetlb_page_range() does a get_page()
on every hugepage.
This is a violation of the shared hugepmd pagetable protocol and creates
additional referenced
to the hugepages causing a leak when the unmap of the VMA occurs.   We
can skip the entire
replication of the ptes when the hugepage pagetables are shared.

The attached patch skips copying the ptes and the get_page() calls if
the hugetlbpage pagetable
is shared.   Fixes BZ 428612 and this patch was send upstream and ACK'd
there.

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0a80f51..e4157ca 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -369,6 +369,9 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 		dst_pte = huge_pte_alloc(dst, addr);
 		if (!dst_pte)
 			goto nomem;
+		/* if the page table is shared dont copy or take references */
+		if (dst_pte == src_pte)
+			continue;
 		spin_lock(&dst->page_table_lock);
 		spin_lock(&src->page_table_lock);
 		if (!pte_none(*src_pte)) {