Re: [PATCH] hugetlbfs: lockdep annotate root inode properly

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 8 Mar 2012 15:19:27 -0600
Tyler Hicks <tyhicks@canonical.com> wrote:

> > 
> > 
> > Sigh.  Was lockdep_annotate_inode_mutex_key() sufficiently
> > self-explanatory to justify leaving it undocumented?
> > 
> > <goes off and reads e096d0c7e2e>
> > 
> > OK, the patch looks correct given the explanation in e096d0c7e2e, but
> > I'd like to understand why it becomes necessary only now.
> > 
> > > NOTE: This patch also require 
> > > http://thread.gmane.org/gmane.linux.file-systems/58795/focus=59565
> > > to remove the lockdep warning
> > 
> > And that patch has been basically ignored.
> 
> Al commented on it here:
> 
> https://lkml.org/lkml/2012/2/16/518
> 
> He said that while my patch is correct, taking i_mutex inside mmap_sem
> is still wrong.

OK, thanks, yup.  Taking i_mutex in file_operations.mmap() is wrong.

Is hugetlbfs actually deadlockable because of this, or is it the case
that the i_mutex->mmap_sem ordering happens to never happen for this
filesystem?  Although we shouldn't go and create incompatible lock
ranking rules for different filesystems!

So we need to pull the i_mutex out of hugetlbfs_file_mmap().  What's it
actually trying to do in there?  If we switch to
i_size_read()/i_size_write() then AFAICT the problem comes down to
hugetlb_reserve_pages().

hugetlb_reserve_pages() fiddles with i_mapping->private_list and the fs
owns private_list and is free to use a lock other than i_mutex to
protect it.  (In fact i_mapping.private_lock is the usual lock for
private_list).



So from a quick scan here I'm thinking that a decent fix is to remove
the i_mutex locking from hugetlbfs_file_mmap(), switch
hugetlbfs_file_mmap() to i_size_read/write then use a hugetlb-private
lock to protect i_mapping->private_list.  region_chg() will do
GFP_KERNEL allocations under that lock, so some care is needed.

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