Re: [PATCH v3 02/14] dm: add ->copy_from_iter() dax operation support

["Kani, Toshimitsu" <toshi.kani@hpe.com>]

On Fri, 2017-06-09 at 13:23 -0700, Dan Williams wrote:
> Allow device-mapper to route copy_from_iter operations to the
> per-target implementation. In order for the device stacking to work
> we
> need a dax_dev and a pgoff relative to that device. This gives each
> layer of the stack the information it needs to look up the operation
> pointer for the next level.
> 
> This conceptually allows for an array of mixed device drivers with
> varying copy_from_iter implementations.
> 
> Cc: Toshi Kani <toshi.kani@hpe.com>
> Reviewed-by: Mike Snitzer <snitzer@redhat.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

I was worried about possible overhead with additional stub calls, but
it looks fine with a single thread fio write test with direct=1.

 92.62%  [kernel.kallsyms]   [k] __copy_user_nocache
  0.04%  [kernel.kallsyms]   [k] entry_SYSCALL_64_fastpath
  0.08%  libpthread-2.22.so  [.] __GI___libc_write
  0.01%  [kernel.kallsyms]   [k] sys_write
  0.02%  [kernel.kallsyms]   [k] vfs_write
  0.02%  [kernel.kallsyms]   [k] __vfs_write
  0.02%  [kernel.kallsyms]   [k] ext4_file_write_iter
  0.02%  [kernel.kallsyms]   [k] dax_iomap_rw
  0.03%  [kernel.kallsyms]   [k] iomap_apply
  0.04%  [kernel.kallsyms]   [k] dax_iomap_actor
  0.01%  [kernel.kallsyms]   [k] dax_copy_from_iter
  0.01%  [kernel.kallsyms]   [k] dm_dax_copy_from_iter
  0.01%  [kernel.kallsyms]   [k] linear_dax_copy_from_iter
  0.03%  [kernel.kallsyms]   [k] copy_from_iter_flushcache
  0.00%  [kernel.kallsyms]   [k] pmem_copy_from_iter

Multi-thread fio test hits hard in inode_lock(), no contention from the
dm-layer.

Reviewed-by: Toshi Kani <toshi.kani@hpe.com>

Thanks,
-Toshi