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

[Andy Lutomirski]

On Fri, May 29, 2015 at 11:19 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Fri, May 29, 2015 at 8:03 AM, Toshi Kani <toshi.kani@xxxxxx> 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@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.
>
> 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?

--Andy
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