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

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Message-ID: <20170726172011.GA30142@infradead.org>
Date:   Wed, 26 Jul 2017 10:20:11 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "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 10:11:08AM -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.

> 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.