BLOCK selects DAX (was: Re: dax: introduce dax_direct_access())

[Geert Uytterhoeven]

Hi Dan,

On Sat, May 6, 2017 at 4:21 AM, Linux Kernel Mailing List
<linux-kernel@xxxxxxxxxxxxxxx> wrote:
> Web:        https://git.kernel.org/torvalds/c/b0686260fecaa924d8eff2ace94bee70506bc308
> Commit:     b0686260fecaa924d8eff2ace94bee70506bc308
> Parent:     d8f07aee3f2fd959878bf614d4e984900018eb9e
> Refname:    refs/heads/master
> Author:     Dan Williams <dan.j.williams@xxxxxxxxx>
> AuthorDate: Thu Jan 26 20:37:35 2017 -0800
> Committer:  Dan Williams <dan.j.williams@xxxxxxxxx>
> CommitDate: Thu Apr 20 11:57:52 2017 -0700
>
>     dax: introduce dax_direct_access()
>
>     Replace bdev_direct_access() with dax_direct_access() that uses
>     dax_device and dax_operations instead of a block_device and
>     block_device_operations for dax. Once all consumers of the old api have
>     been converted bdev_direct_access() will be deleted.
>
>     Given that block device partitioning decisions can cause dax page
>     alignment constraints to be violated this also introduces the
>     bdev_dax_pgoff() helper. It handles calculating a logical pgoff relative
>     to the dax_device and also checks for page alignment.
>
>     Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

This looks like a fairly innocent change...

> ---
>  block/Kconfig          |  1 +
>  drivers/dax/super.c    | 39 +++++++++++++++++++++++++++++++++++++++
>  fs/block_dev.c         | 14 ++++++++++++++
>  include/linux/blkdev.h |  1 +
>  include/linux/dax.h    |  2 ++
>  5 files changed, 57 insertions(+)
>
> diff --git a/block/Kconfig b/block/Kconfig
> index e9f780f815f5..93da7fc3f254 100644
> --- a/block/Kconfig
> +++ b/block/Kconfig
> @@ -6,6 +6,7 @@ menuconfig BLOCK
>         default y
>         select SBITMAP
>         select SRCU
> +       select DAX
>         help
>          Provide block layer support for the kernel.

... but it is not: now DAX is always enabled if you use block devices.

    $ bloat-o-meter vmlinux.nodax vmlinux.dax  | head
    add/remove: 59/0 grow/shrink: 10/1 up/down: 6048/-8 (6040)
    function                                     old     new   delta
    dax_host_list                                  -    4096   +4096
    alloc_dax                                      -     306    +306
    dax_fs_init                                    -     230    +230
    dax_get_by_host                                -     128    +128
    dax_sops                                       -     100    +100
    dax_srcu                                       -      90     +90
    dax_direct_access                              -      80     +80
    bdev_dax_pgoff                                 -      80     +80

Does all block device access really needs DAX enabled?

Furthermore:

      Maximum number of Device-DAX instances (NR_DEV_DAX) [32768] (NEW) ?

    There is no help available for this option.
    Symbol: NR_DEV_DAX [=32768]
    Type  : integer
    Range : [256 2147483647]
    Prompt: Maximum number of Device-DAX instances
      Location:
        -> Device Drivers
          -> DAX: direct access to differentiated memory (DAX [=y])
      Defined at drivers/dax/Kconfig:31
      Depends on: DAX [=y]

What should I answer here?
"256" already sounds like a large number to me, "32768" sounds huge, and
"2147483647" sounds insane? But perhaps this doesn't really mean what I
would expect...

Can you please add some help for this option?

Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds