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xfs: introduce mmap/truncate lock
Right now we cannot serialise mmap against truncate or hole punch sanely. ->page_mkwrite is not able to take locks that the read IO path normally takes (i.e. the inode iolock) because that could result in lock inversions (read - iolock - page fault - page_mkwrite - iolock) and so we cannot use an IO path lock to serialise page write faults against truncate operations. Instead, introduce a new lock that is used *only* in the ->page_mkwrite path that is the equivalent of the iolock. The lock ordering in a page fault is i_mmaplock -> page lock -> i_ilock, and so in truncate we can i_iolock -> i_mmaplock and so lock out new write faults during the process of truncation. Because i_mmap_lock is outside the page lock, we can hold it across all the same operations we hold the i_iolock for. The only difference is that we never hold the i_mmaplock in the normal IO path and so do not ever have the possibility that we can page fault inside it. Hence there are no recursion issues on the i_mmap_lock and so we can use it to serialise page fault IO against inode modification operations that affect the IO path. This patch introduces the i_mmaplock infrastructure, lockdep annotations and initialisation/destruction code. Use of the new lock will be in subsequent patches. Signed-off-by:Dave Chinner <dchinner@redhat.com> Reviewed-by: