Hi all,
Now I am trying to handle AIO DIO with O_SYNC using extent status tree in ext4.
After applied Christoph's patch series, O_SYNC semantics in ext4 will be broken.
This problem can be fixed using extent status tree. But we will get a deadlock
because i_mutex needs to be taken in ext4_sync_file() and then it will wait on
i_unwritten==0. So let's consider what happends after applied Christoph's
patches and using extent status tree to ensure AIO DIO with O_SYNC semantics.
ext4_ext_direct_IO: ext4_ind_direct_IO:
->ext4_file_write()
->mutex_lock(i_mutex)
->ext4_ind_direct_IO()
[if this is an append dio]
->mutex_unlock(i_mutex)
->ext4_file_write()
->mutex_lock(i_mutex)
->ext4_ext_direct_IO()
->mutex_unlock(i_mutex)
->generic_write_sync()
->ext4_sync_file()
->mutex_lock(i_mutex)
->ext4_flush_unwritten_io()
->ext4_do_flush_complete_IO()
[there is empty list]
->ext4_unwritten_wait()
[wait on i_unwritten==0 because
in ext4_ext_direct_IO i_unwritten
has been increased]
kworkd:
->dio_complete()
->ext4_end_dio()
->ext4_es_convert_unwritten_extents()
[convert unwritten extents in status
tree to ensure O_SYNC semantics]
->ext4_add_complete_io()
->generic_write_sync()
->ext4_sync_file()
->mutex_lock(i_mutex)
[*DEADLOCK*]
Thus all we need to do is do not wait on i_unwritten==0. But, as this
commit (c278531d) described, there is a time window that integrity is
broken. So we need to call end_page_writeback() after converting
unwritten extents in ext4_end_io(). However, if we call end_page_writeback()
after conversion has been done in ext4_end_io(), we will get another deadlock
because in ext4_convert_unwritten_extents() we need to start a journal and it is
possible to cause a journal commit. At the time if ext4_write_begin() is
called, it also will start a journal and then it will wait on writeback in
grab_cache_page_write_begin().
Now I have an idea to solve this problem. We start a journal before submitting
an io request rather than start it in ext4_convert_unwritten_extents(). The
reason of starting a journal in ext4_convert_unwritten_extents() is that we need
to calculate credits for journal. But as far as I understand the credits is not
increased in this function because we have splitted extents before submitting
this io request. A 'handle_t *handle' will be added into ext4_io_end_t, and it
will be used in ext4_convert_unwritten_extents(). Then we can avoid to
trigger a journal commit when starting a journal.
Hope my description is clear. Any comments or feedbacks are always welcome.
Jan, I don't know whether you have begun to try to fix this problem or not. If
there has an update, please let me know.
Thanks,
- Zheng
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