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

From: Jan Kara
Date: Mon Nov 16 2015 - 08:37:28 EST

On Fri 13-11-15 18:32:40, Dan Williams wrote:
> On Fri, Nov 13, 2015 at 4:43 PM, Andreas Dilger <adilger@xxxxxxxxx> wrote:
> > On Nov 13, 2015, at 5:20 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >>
> >> 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.
> >
> > If the upper level filesystem gets an error when submitting a flush
> > request, then it assumes the underlying hardware is broken and cannot
> > be as aggressive in IO submission, but instead has to wait for in-flight
> > IO to complete.
> Upper level filesystems won't get errors when the driver does not
> support flush. Those requests are ended cleanly in
> generic_make_request_checks(). Yes, the fs still needs to wait for
> outstanding I/O to complete but in the case of pmem all I/O is
> synchronous. There's never anything to await when flushing at the
> pmem driver level.
> > Since FUA/FLUSH is basically a no-op for pmem devices,
> > it doesn't make sense _not_ to support this functionality.
> Seems to be a nop either way. Given that DAX may lead to dirty data
> pending to the device in the cpu cache that a REQ_FLUSH request will
> not touch, its better to leave it all to the mm core to handle. I.e.
> it doesn't make sense to call the driver just for two instructions
> (sfence + pcommit) when the mm core is taking on the cache flushing.
> Either handle it all in the mm or the driver, not a mixture.

So I think REQ_FLUSH requests *must* end up doing sfence + pcommit because
e.g. journal writes going through block layer or writes done through
dax_do_io() must be on permanent storage once REQ_FLUSH request finishes
and the way driver does IO doesn't guarantee this, does it?

Jan Kara <jack@xxxxxxxx>
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