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