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

[Dan Williams <dan.j.williams@intel.com>]

On Fri, May 29, 2015 at 11:34 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Fri, May 29, 2015 at 11:19 AM, Dan Williams <dan.j.williams@intel.com> wrote:
>> On Fri, May 29, 2015 at 8:03 AM, Toshi Kani <toshi.kani@hp.com> wrote:
>>> On Fri, 2015-05-29 at 07:43 -0700, Dan Williams wrote:
>>>> On Fri, May 29, 2015 at 2:11 AM, Borislav Petkov <bp@alien8.de> 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@hp.com>
>>>> >> ---
>>>> >>  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.
>>
>> Yes, but now if the caller was expecting one memory type and gets
>> another one that is something I think the driver would want to know.
>> At a minimum I don't think we want to get emails about pmem driver
>> performance problems when someone's platform is silently degrading WB
>> to UC for example.
>>
>>> DAX uses vm_insert_mixed(), which does not even check the existing map
>>> type to the physical address.
>>
>> Right, I think that's a problem...
>>
>>>> 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.
>>
>> Hmm, yes you're right, it seems those patches did not change the
>> implementation to use ioremap_cache()... which happens to not be
>> implemented on all architectures.  I'll take a look.
>
> Whoa, there!  Why would we use non-temporal stores to WB memory to
> access persistent memory?  I can see two reasons not to:
>
> 1. As far as I understand it, non-temporal stores to WT should have
> almost identical performance.
>
> 2. Is there any actual architectural guarantee that it's safe to have
> a WB mapping that we use like that?  By my reading of the manual,
> MOVNTDQA (as a write to pmem); SFENCE; PCOMMIT; SFENCE on uncached
> memory should be guaranteed to do a durable write.  On the other hand,
> it's considerably less clear to me that the same sequence to WB memory
> is safe -- aren't we supposed to stick a CLWB or CLFLUSHOPT in there,
> too, on WB memory?  In other words, is there any case in which
> MOVNTDQA or similar acting on a WB mapping could result in a dirty
> cache line?

Depends, see the note in 10.4.6.2, "Some older CPU implementations
(e.g., Pentium M) allowed addresses being written with a non-temporal
store instruction to be updated in-place if the memory type was not WC
and line was already in the cache."  The expectation is that
boot_cpu_has(X86_FEATURE_PCOMMIT) is false on such a cpu so we'll
fallback to not using non-temporal stores.

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