linux-kernel - Re: [PATCH v8 1/1] f2fs: dax: implement direct access

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Message-ID: <CAPcyv4hdk8XyYJ5pYvAgUZRoE-SzACNprQm1xaM1KUqBONC8Qw@mail.gmail.com>
Date:   Wed, 26 Jul 2017 12:16:11 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        sunqiuyang <sunqiuyang@...wei.com>
Subject: Re: [PATCH v8 1/1] f2fs: dax: implement direct access

On Wed, Jul 26, 2017 at 10:20 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Wed, Jul 26, 2017 at 10:11:08AM -0700, Dan Williams wrote:
>> Until HMAT came along we had no data in the kernel how to pick a sane
>> default, but we could now very easily make a "if pmem performance <
>> dram, disable dax by default" policy in the kernel.
>
> I'd rather do it the other way around - if HMAT is present and
> pmem performance >= dram use dax.  Else require the explicit -o dax
> for now to enable it.  If an explicit -o nodax is specified disable
> DAX even if HMAT says it is faster.

Silently turn on DAX if HMAT says its ok? I think we would instead
want a "-o autodax" for that case and then "-o dax" and "-o nodax" for
the force cases.

>> The question for this patch is do we want to add yet another
>> filesystem that adds "-o dax" or require use of per-inode flags to
>> enable dax.
>
> Please stick to the mount option.  After spending a lot of time with
> DAX and various memory techologies I'm pretty confident that the inode
> flag is the wrong thing to do.

I think it's easier to administer than the dax mount option. If
someone wants dax on only in a sub-tree they can set the flag on that
parent directory and have a policy in dax filesystems that children
inherit the dax policy from the parent. That seems a better
administrative model than trying to get it all right globally at mount
time.