Re: Subtle races between DAX mmap fault and write path

Dan Williams
2016-07-29T14:44:29Z

On Thu, Jul 28, 2016 at 7:21 PM, Dave Chinner <david@fromorbit.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.