Re: [PATCH 2/2] libnvdimm: don't flush power-fail protected CPU caches

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Tue, Jun 05, 2018 at 02:20:38PM -0700, Dan Williams wrote:
> On Tue, Jun 5, 2018 at 1:58 PM, Ross Zwisler
> <ross.zwisler@linux.intel.com> wrote:
> > This commit:
> >
> > 5fdf8e5ba566 ("libnvdimm: re-enable deep flush for pmem devices via fsync()")
> >
> > intended to make sure that deep flush was always available even on
> > platforms which support a power-fail protected CPU cache.  An unintended
> > side effect of this change was that we also lost the ability to skip
> > flushing CPU caches on those power-fail protected CPU cache.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> > Fixes: 5fdf8e5ba566 ("libnvdimm: re-enable deep flush for pmem devices via fsync()")
> > ---
> >  drivers/dax/super.c   | 20 +++++++++++++++++++-
> >  drivers/nvdimm/pmem.c |  2 ++
> >  include/linux/dax.h   |  9 +++++++++
> >  3 files changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> > index c2c46f96b18c..457e0bb6c936 100644
> > --- a/drivers/dax/super.c
> > +++ b/drivers/dax/super.c
> > @@ -152,6 +152,8 @@ enum dax_device_flags {
> >         DAXDEV_ALIVE,
> >         /* gate whether dax_flush() calls the low level flush routine */
> >         DAXDEV_WRITE_CACHE,
> > +       /* only flush the CPU caches if they are not power fail protected */
> > +       DAXDEV_FLUSH_ON_SYNC,
> >  };
> >
> >  /**
> > @@ -283,7 +285,8 @@ EXPORT_SYMBOL_GPL(dax_copy_from_iter);
> >  void arch_wb_cache_pmem(void *addr, size_t size);
> >  void dax_flush(struct dax_device *dax_dev, void *addr, size_t size)
> >  {
> > -       if (unlikely(!dax_write_cache_enabled(dax_dev)))
> > +       if (unlikely(!dax_write_cache_enabled(dax_dev)) ||
> > +                       !dax_flush_on_sync_enabled(dax_dev))
> 
> This seems backwards. I think we should teach the pmem driver to still
> issue deep flush even when dax_write_cache_enabled() is false.

That does still happen.  Deep flush is essentially controlled by the 'wbc'
variable in pmem_attach_disk(), which we use to set blk_queue_write_cache().
My understanding is that this causes the block layer to send down
REQ_FUA/REQ_PREFLUSH BIOs, and it's in response to these that we do a deep
flush via nvdimm_flush().  Whether this happens is totally up to the device's
write cache setting, and doesn't look at whether the platform has
flush-on-fail CPU caches.

This does bring up another wrinkle, though: we export a write_cache sysfs
entry that you can use to change the write cache setting of a namespace:

i.e.:
	/sys/bus/nd/devices/pfn0.1/block/pmem0/dax/write_cache

This changes whether or not the DAXDEV_WRITE_CACHE flag is set, but does *not*
change whether the block queue says it supports a write cache
(blk_queue_write_cache()).  So, the sysfs entry ends up controlling whether or
not we do CPU cache flushing via DAX, but does not do anything with the deep
flush code.

I'm guessing this should be fixed?  I'll go take a look...
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