On Thu, Oct 06, 2016 at 03:34:24PM -0600, Ross Zwisler wrote:

Interesting - adding iomap_end() calls to the DAX PTE fault handler causes an
AA deadlock because we try and retake ei->dax_sem.  We take dax_sem in
ext2_dax_fault() before calling into the DAX code, then if we end up going
through the error path in ext2_iomap_end(), we call 
  ext2_write_failed()
    ext2_truncate_blocks()
      dax_sem_down_write()

Where we try and take dax_sem again.  This error path is really only valid for
I/O operations, but we happen to call it for page faults because 'written' in
ext2_iomap_end() is just 0.

So...how should we handle this?  A few ideas:

1) Just continue to omit the calls to iomap_end() in the DAX page fault
handlers for now, and add them when there is useful work to be done in one of
the filesystems.

2) Add an IOMAP_FAULT flag to the flags passed into iomap_begin() and
iomap_end() so make it explicit that we are calling as part of a fault handler
and not an I/O operation, and use this to adjust the error handling in
ext2_iomap_end().

3) Just work around the existing error handling in ext2_iomap_end() by either
unsetting IOMAP_WRITE or by setting 'written' to the size of the fault.

For #2 or #3, probably add a comment explaining the deadlock and why we need
to never call ext2_write_failed() while handling a page fault.

Thoughts?

Never mind, #3 it is, I think it was just a plain bug to call iomap_end() with
'length' != 'written'.