Re: [PATCH] hugetlbfs: move resv_map to hugetlbfs_inode_info

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

On 4/11/19 9:02 PM, Yufen Yu wrote:
> Commit 58b6e5e8f1ad ("hugetlbfs: fix memory leak for resv_map")
...
> However, for inode mode that is 'S_ISBLK', hugetlbfs_evict_inode() may
> free or modify i_mapping->private_data that is owned by bdev inode,
> which is not expected!
...
> We fix the problem by moving resv_map to hugetlbfs_inode_info. It may
> be more reasonable.

Your patches force me to consider these potential issues.  Thank you!

The root of all these problems (including the original leak) is that the
open of a block special inode will result in bd_acquire() overwriting the
value of inode->i_mapping.  Since hugetlbfs inodes normally contain a
resv_map at inode->i_mapping->private_data, a memory leak occurs if we do
not free the initially allocated resv_map.  In addition, when the
inode is evicted/destroyed inode->i_mapping may point to an address space
not associated with the hugetlbfs inode.  If code assumes inode->i_mapping
points to hugetlbfs inode address space at evict time, there may be bad
data references or worse.

This specific part of the patch made me think,

> @@ -497,12 +497,15 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>  static void hugetlbfs_evict_inode(struct inode *inode)
>  {
>  	struct resv_map *resv_map;
> +	struct hugetlbfs_inode_info *info = HUGETLBFS_I(inode);
>  
>  	remove_inode_hugepages(inode, 0, LLONG_MAX);
> -	resv_map = (struct resv_map *)inode->i_mapping->private_data;
> +	resv_map = info->resv_map;
>  	/* root inode doesn't have the resv_map, so we should check it */
> -	if (resv_map)
> +	if (resv_map) {
>  		resv_map_release(&resv_map->refs);
> +		info->resv_map = NULL;
> +	}
>  	clear_inode(inode);
>  }

If inode->i_mapping may not be associated with the hugetlbfs inode, then
remove_inode_hugepages() will also have problems.  It will want to operate
on the address space associated with the inode.  So, there are more issues
than just the resv_map.  When I looked at the first few lines of
remove_inode_hugepages(), I was surprised to see:

	struct address_space *mapping = &inode->i_data;

So remove_inode_hugepages is explicitly using the original address space
that is embedded in the inode.  As a result, it is not impacted by changes
to inode->i_mapping.  Using git history I was unable to determine why
remove_inode_hugepages is the only place in hugetlbfs code doing this.

With this in mind, a simple change like the following will fix the original
leak issue as well as the potential issues mentioned in this patch.

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 53ea3cef526e..9f0719bad46f 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -511,6 +511,11 @@ static void hugetlbfs_evict_inode(struct inode *inode)
 {
 	struct resv_map *resv_map;
 
+	/*
+	 * Make sure we are operating on original hugetlbfs address space.
+	 */
+	inode->i_mapping = &inode->i_data;
+
 	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 */


I don't know why hugetlbfs code would ever want to operate on any address
space but the one embedded within the inode.  However, my uderstanding of
the vfs layer is somewhat limited.  I'm wondering if the hugetlbfs code
(helper routines mostly) should perhaps use &inode->i_data instead of
inode->i_mapping.  Does it ever make sense for hugetlbfs code to operate
on inode->i_mapping if inode->i_mapping != &inode->i_data ?
-- 
Mike Kravetz