linux-kernel - Re: BLOCK selects DAX (was: Re: dax: introduce dax_direct

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Date:   Sun, 7 May 2017 21:09:58 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...com>, linux-block@...r.kernel.org
Subject: Re: BLOCK selects DAX (was: Re: dax: introduce dax_direct_access())

Hi Dan,

On Sun, May 7, 2017 at 8:26 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Sun, May 7, 2017 at 9:20 AM, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> On Sat, May 6, 2017 at 4:21 AM, Linux Kernel Mailing List
>> <linux-kernel@...r.kernel.org> wrote:
>>> Web:        https://git.kernel.org/torvalds/c/b0686260fecaa924d8eff2ace94bee70506bc308
>>> Commit:     b0686260fecaa924d8eff2ace94bee70506bc308
>>> Parent:     d8f07aee3f2fd959878bf614d4e984900018eb9e
>>> Refname:    refs/heads/master
>>> Author:     Dan Williams <dan.j.williams@...el.com>
>>> AuthorDate: Thu Jan 26 20:37:35 2017 -0800
>>> Committer:  Dan Williams <dan.j.williams@...el.com>
>>> 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@...el.com>
>>
>> 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
>
> Whoops, yes, that should be "select DAX if FS_DAX". We don't need the
> DAX core if we're never going to mount a DAX capable filesystem.

Good, thanks!

>> 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?
>
> Will do. The number gets passed directly to the @count parameter of

OK, thanks!

> alloc_chrdev_region().

BTW, I saw that. But it still didn't give me a clue about a useful value.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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