[PATCH 0/6] ext4: Clean up io_end handling for AIO DIO

[Jan Kara <jack@suse.cz>]

Hello,

Recently I had a look at how io_ends are handled for AIO DIO. When doing AIO
DIO we allocate io_end and attach it to inode. ext4_ext_map_blocks() then
fetches io_end from inode and operates on it. This scheme is somewhat fragile
because it relies on the fact that we hold i_mutex whenever we need io_end
which is non-obvious dependency (if it was not for io_end handling, i_data_sem
would be enough). dioread_nolock mode broke this assumption and was corrupting
io_end pointers until recently which somewhat demostrates my point.

This patch set converts ext4 to allocate io_end only once we spot unwritten
extent during direct IO write and uses bh->b_private and dio->private
to store it in the same fashion as XFS. That way passing of io_end pointer
happens via IO-local context and is thus independent of any locks.

The core of the change is in patch 5/6, the rest are related cleanups and
preparations for the change.

Patches pass xfstests in normal and dioread_nolock mode. Patch 5/6 conflicts
with changes of DIO completion on error however resolving these conflicts
should be straightforward.

								Honza