Re: [PATCH] hugetlbfs: move resv_map to hugetlbfs_inode_info

[Michal Hocko <mhocko@kernel.org>]

On Mon 15-04-19 06:16:15, Naoya Horiguchi wrote:
> On Fri, Apr 12, 2019 at 04:40:01PM -0700, Mike Kravetz wrote:
> > 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.
> 
> Let me ask a kind of elementary question: is there any good reason/purpose
> to create and use block special files on hugetlbfs?  I never heard about
> such usecases.  I guess that the conflict of the usage of ->i_mapping is
> discovered recently and that's because block special files on hugetlbfs are
> just not considered until recently or well defined.  So I think that we might
> be better to begin with defining it first.

A absolutely agree. Hugetlbfs is overly complicated even without that.
So if this is merely "we have tried it and it has blown up" kinda thing
then just refuse the create blockdev files or document it as undefined.
You need a root to do so anyway.
-- 
Michal Hocko
SUSE Labs