linux-ext4 - Re: Subtle races between DAX mmap fault and write path

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Date:	Fri, 29 Jul 2016 07:44:25 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jan Kara <jack@...e.cz>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	XFS Developers <xfs@....sgi.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: Subtle races between DAX mmap fault and write path

On Thu, Jul 28, 2016 at 7:21 PM, Dave Chinner <david@...morbit.com> wrote:
> On Thu, Jul 28, 2016 at 10:10:33AM +0200, Jan Kara wrote:
>> On Thu 28-07-16 08:19:49, Dave Chinner wrote:
[..]
>> So DAX doesn't need flushing to maintain consistent view of the data but it
>> does need flushing to make sure fsync(2) results in data written via mmap
>> to reach persistent storage.
>
> I thought this all changed with the removal of the pcommit
> instruction and wmb_pmem() going away.  Isn't it now a platform
> requirement now that dirty cache lines over persistent memory ranges
> are either guaranteed to be flushed to persistent storage on power
> fail or when required by REQ_FLUSH?

No, nothing automates cache flushing.  The path of a write is:

cpu-cache -> cpu-write-buffer -> bus -> imc -> imc-write-buffer -> media

The ADR mechanism and the wpq-flush facility flush data thorough the
imc (integrated memory controller) to media.  dax_do_io() gets writes
to the imc, but we still need a posted-write-buffer flush mechanism to
guarantee data makes it out to media.


> https://lkml.org/lkml/2016/7/9/131
>
> And part of that is the wmb_pmem() calls are going away?
>
> https://lkml.org/lkml/2016/7/9/136
> https://lkml.org/lkml/2016/7/9/140
>
> i.e. fsync on pmem only needs to take care of writing filesystem
> metadata now, and the pmem driver handles the rest when it gets a
> REQ_FLUSH bio from fsync?
>
> https://lkml.org/lkml/2016/7/9/134
>
> Or have we somehow ended up with the fucked up situation where
> dax_do_io() writes are (effectively) immediately persistent and
> untracked by internal infrastructure, whilst mmap() writes
> require internal dirty tracking and fsync() to flush caches via
> writeback?

dax_do_io() writes are not immediately persistent.  They bypass the
cpu-cache and cpu-write-bufffer and are ready to be flushed to media
by REQ_FLUSH or power-fail on an ADR system.
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