Re: [PATCH v10 12/12] drivers/block/pmem: Map NVDIMM with ioremap_wt()

[Toshi Kani]

On Fri, 2015-05-29 at 07:43 -0700, Dan Williams wrote:
> On Fri, May 29, 2015 at 2:11 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> > On Wed, May 27, 2015 at 09:19:04AM -0600, Toshi Kani wrote:
> >> The pmem driver maps NVDIMM with ioremap_nocache() as we cannot
> >> write back the contents of the CPU caches in case of a crash.
> >>
> >> This patch changes to use ioremap_wt(), which provides uncached
> >> writes but cached reads, for improving read performance.
> >>
> >> Signed-off-by: Toshi Kani <toshi.kani@xxxxxx>
> >> ---
> >>  drivers/block/pmem.c |    4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/block/pmem.c b/drivers/block/pmem.c
> >> index eabf4a8..095dfaa 100644
> >> --- a/drivers/block/pmem.c
> >> +++ b/drivers/block/pmem.c
> >> @@ -139,11 +139,11 @@ static struct pmem_device *pmem_alloc(struct device *dev, struct resource *res)
> >>       }
> >>
> >>       /*
> >> -      * Map the memory as non-cachable, as we can't write back the contents
> >> +      * Map the memory as write-through, as we can't write back the contents
> >>        * of the CPU caches in case of a crash.
> >>        */
> >>       err = -ENOMEM;
> >> -     pmem->virt_addr = ioremap_nocache(pmem->phys_addr, pmem->size);
> >> +     pmem->virt_addr = ioremap_wt(pmem->phys_addr, pmem->size);
> >>       if (!pmem->virt_addr)
> >>               goto out_release_region;
> >
> > Dan, Ross, what about this one?
> >
> > ACK to pick it up as a temporary solution?
> 
> I see that is_new_memtype_allowed() is updated to disallow some
> combinations, but the manual seems to imply any mixing of memory types
> is unsupported.  Which worries me even in the current code where we
> have uncached mappings in the driver, and potentially cached DAX
> mappings handed out to userspace.

is_new_memtype_allowed() is not to allow some combinations of mixing of
memory types.  When it is allowed, the requested type of ioremap_xxx()
is changed to match with the existing map type, so that mixing of memory
types does not happen.

DAX uses vm_insert_mixed(), which does not even check the existing map
type to the physical address.

> A general quibble separate from this patch is that we don't have a way
> of knowing if ioremap() will reject or change our requested memory
> type.  Shouldn't the driver be explicitly requesting a known valid
> type in advance?

I agree we need a solution here.

> Lastly we now have the PMEM API patches from Ross out for review where
> he is assuming cached mappings with non-temporal writes:
> https://lists.01.org/pipermail/linux-nvdimm/2015-May/000929.html.
> This gives us WC semantics on writes which I believe has the nice
> property of reducing the number of write transactions to memory.
> Also, the numbers in the paper seem to be assuming DAX operation, but
> this ioremap_wt() is in the driver and typically behind a file system.
> Are the numbers relevant to that usage mode?

I have not looked into the Ross's changes yet, but they do not seem to
replace the use of ioremap_nocache().  If his changes can use WB type
reliably, yes, we do not need a temporary solution of using ioremap_wt()
in this driver.

Thanks,
-Toshi

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