[PATCH] hugetlbfs: always use address space in inode for resv_map pointer

[Mike Kravetz <mike.kravetz@oracle.com>]

Continuing discussion about commit 58b6e5e8f1ad ("hugetlbfs: fix memory
leak for resv_map") brought up the issue that inode->i_mapping may not
point to the address space embedded within the inode at inode eviction
time.  The hugetlbfs truncate routine handles this by explicitly using
inode->i_data.  However, code cleaning up the resv_map will still use
the address space pointed to by inode->i_mapping.  Luckily, private_data
is NULL for address spaces in all such cases today but, there is no
guarantee this will continue.

Change all hugetlbfs code getting a resv_map pointer to explicitly get
it from the address space embedded within the inode.  In addition, add
more comments in the code to indicate why this is being done.

Reported-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 fs/hugetlbfs/inode.c | 11 +++++++++--
 mm/hugetlb.c         | 19 ++++++++++++++++++-
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 9285dd4f4b1c..cbc649cd1722 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -499,8 +499,15 @@ static void hugetlbfs_evict_inode(struct inode *inode)
 	struct resv_map *resv_map;
 
 	remove_inode_hugepages(inode, 0, LLONG_MAX);
-	resv_map = (struct resv_map *)inode->i_mapping->private_data;
-	/* root inode doesn't have the resv_map, so we should check it */
+
+	/*
+	 * Get the resv_map from the address space embedded in the inode.
+	 * This is the address space which points to any resv_map allocated
+	 * at inode creation time.  If this is a device special inode,
+	 * i_mapping may not point to the original address space.
+	 */
+	resv_map = (struct resv_map *)(&inode->i_data)->private_data;
+	/* Only regular and link inodes have associated reserve maps */
 	if (resv_map)
 		resv_map_release(&resv_map->refs);
 	clear_inode(inode);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6cdc7b2d9100..b30e97b0ef37 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -740,7 +740,15 @@ void resv_map_release(struct kref *ref)
 
 static inline struct resv_map *inode_resv_map(struct inode *inode)
 {
-	return inode->i_mapping->private_data;
+	/*
+	 * At inode evict time, i_mapping may not point to the original
+	 * address space within the inode.  This original address space
+	 * contains the pointer to the resv_map.  So, always use the
+	 * address space embedded within the inode.
+	 * The VERY common case is inode->mapping == &inode->i_data but,
+	 * this may not be true for device special inodes.
+	 */
+	return (struct resv_map *)(&inode->i_data)->private_data;
 }
 
 static struct resv_map *vma_resv_map(struct vm_area_struct *vma)
@@ -4477,6 +4485,11 @@ int hugetlb_reserve_pages(struct inode *inode,
 	 * called to make the mapping read-write. Assume !vma is a shm mapping
 	 */
 	if (!vma || vma->vm_flags & VM_MAYSHARE) {
+		/*
+		 * resv_map can not be NULL as hugetlb_reserve_pages is only
+		 * called for inodes for which resv_maps were created (see
+		 * hugetlbfs_get_inode).
+		 */
 		resv_map = inode_resv_map(inode);
 
 		chg = region_chg(resv_map, from, to);
@@ -4568,6 +4581,10 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
 	struct hugepage_subpool *spool = subpool_inode(inode);
 	long gbl_reserve;
 
+	/*
+	 * Since this routine can be called in the evict inode path for all
+	 * hugetlbfs inodes, resv_map could be NULL.
+	 */
 	if (resv_map) {
 		chg = region_del(resv_map, start, end);
 		/*
-- 
2.20.1