Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling

[Dan Williams <dan.j.williams@xxxxxxxxx>]

On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> Currently the PMEM driver doesn't accept REQ_FLUSH or REQ_FUA bios.  These
> are sent down via blkdev_issue_flush() in response to a fsync() or msync()
> and are used by filesystems to order their metadata, among other things.
>
> When we get an msync() or fsync() it is the responsibility of the DAX code
> to flush all dirty pages to media.  The PMEM driver then just has issue a
> wmb_pmem() in response to the REQ_FLUSH to ensure that before we return all
> the flushed data has been durably stored on the media.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>

Hmm, I'm not seeing why we need this patch.  If the actual flushing of
the cache is done by the core why does the driver need support
REQ_FLUSH?  Especially since it's just a couple instructions.  REQ_FUA
only makes sense if individual writes can bypass the "drive" cache,
but no I/O submitted to the driver proper is ever cached we always
flush it through to media.