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

From	Jan Kara <jack@suse.cz>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling
Date	2015-11-16 14:40 +0100
Message-ID	<qvsaK-7Us-27@gated-at.bofh.it> (permalink)
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On Fri 13-11-15 18:32:40, Dan Williams wrote:
> On Fri, Nov 13, 2015 at 4:43 PM, Andreas Dilger <adilger@dilger.ca> wrote:
> > On Nov 13, 2015, at 5:20 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> >>
> >> On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler
> >> <ross.zwisler@linux.intel.com> 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@linux.intel.com>
> >>
> >> 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?

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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Thread

[PATCH v2 00/11] DAX fsynx/msync support Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:10 +0100
  [PATCH v2 09/11] ext2: add support for DAX fsync/msync Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:10 +0100
  [PATCH v2 07/11] mm: add find_get_entries_tag() Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:10 +0100
    Re: [PATCH v2 07/11] mm: add find_get_entries_tag() Dave Chinner <david@fromorbit.com> - 2015-11-16 23:50 +0100
      Re: [PATCH v2 07/11] mm: add find_get_entries_tag() Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-17 19:20 +0100
  [PATCH v2 10/11] ext4: add support for DAX fsync/msync Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:10 +0100
  [PATCH v2 11/11] xfs: add support for DAX fsync/msync Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:10 +0100
    Re: [PATCH v2 11/11] xfs: add support for DAX fsync/msync Dave Chinner <david@fromorbit.com> - 2015-11-17 00:20 +0100
      Re: [PATCH v2 11/11] xfs: add support for DAX fsync/msync Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-17 20:10 +0100
        Re: [PATCH v2 11/11] xfs: add support for DAX fsync/msync Dave Chinner <david@fromorbit.com> - 2015-11-20 01:50 +0100
  [PATCH v2 05/11] mm: add follow_pte_pmd() Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:20 +0100
  [PATCH v2 01/11] pmem: add wb_cache_pmem() to the PMEM API Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:20 +0100
  [PATCH v2 08/11] dax: add support for fsync/sync Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:20 +0100
    Re: [PATCH v2 08/11] dax: add support for fsync/sync Dave Chinner <david@fromorbit.com> - 2015-11-17 00:00 +0100
      Re: [PATCH v2 08/11] dax: add support for fsync/sync Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-17 19:40 +0100
  [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:20 +0100
    Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Dan Williams <dan.j.williams@intel.com> - 2015-11-14 01:30 +0100
      Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Andreas Dilger <adilger@dilger.ca> - 2015-11-14 01:50 +0100
        Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Dan Williams <dan.j.williams@intel.com> - 2015-11-14 03:40 +0100
          Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Jan Kara <jack@suse.cz> - 2015-11-16 14:40 +0100
            Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Jan Kara <jack@suse.cz> - 2015-11-16 15:20 +0100
              Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Dan Williams <dan.j.williams@intel.com> - 2015-11-16 18:30 +0100
                Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-16 20:50 +0100
                Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Dan Williams <dan.j.williams@intel.com> - 2015-11-16 21:40 +0100
                Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-17 01:00 +0100
              Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Dave Chinner <david@fromorbit.com> - 2015-11-16 23:20 +0100
                Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-17 00:30 +0100
                Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Dave Chinner <david@fromorbit.com> - 2015-11-17 00:50 +0100
          Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-16 21:10 +0100
            Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Jan Kara <jack@suse.cz> - 2015-11-18 11:50 +0100
              Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-18 17:20 +0100
  [PATCH v2 06/11] mm: add pgoff_mkclean() Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:20 +0100
  [PATCH v2 02/11] mm: add pmd_mkclean() Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:20 +0100
    Re: [PATCH v2 02/11] mm: add pmd_mkclean() Dave Hansen <dave.hansen@intel.com> - 2015-11-14 02:10 +0100
      Re: [PATCH v2 02/11] mm: add pmd_mkclean() Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-17 19:00 +0100
  [PATCH v2 04/11] dax: support dirty DAX entries in radix tree Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-14 01:20 +0100
  Re: [PATCH v2 00/11] DAX fsynx/msync support Jan Kara <jack@suse.cz> - 2015-11-16 15:50 +0100
    Re: [PATCH v2 00/11] DAX fsynx/msync support Dan Williams <dan.j.williams@intel.com> - 2015-11-16 18:00 +0100
      Re: [PATCH v2 00/11] DAX fsynx/msync support Ross Zwisler <ross.zwisler@linux.intel.com> - 2015-11-16 21:10 +0100

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