Re: [RFC 0/2] New MAP_PMEM_AWARE mmap flag

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

On Tue, Feb 23, 2016 at 9:05 AM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> On Tue, Feb 23, 2016 at 08:56:57AM -0800, Dan Williams wrote:
>> On Tue, Feb 23, 2016 at 6:10 AM, Boaz Harrosh <boaz@plexistor.com> wrote:
>> > On 02/23/2016 11:52 AM, Christoph Hellwig wrote:
>> [..]
>> > Please tell me what you find wrong with my approach?
>>
>> Setting aside fs interactions you didn't respond to my note about
>> architectures where the pmem-aware app needs to flush caches due to
>> other non-pmem aware apps sharing the mapping.  Non-temporal stores
>> guaranteeing persistence on their own is an architecture specific
>> feature.  I don't see how we can have a generic support for mixed
>> MAP_PMEM_AWARE / unaware shared mappings when the architecture
>> dependency exists [1].
>>
>> I think Christoph has already pointed out the roadmap.  Get the
>> existing crop of DAX bugs squashed and then *maybe* look at something
>> like a MAP_SYNC to opt-out of userspace needing to call *sync.
>>
>> [1]: 10.4.6.2 Caching of Temporal vs. Non-Temporal Data
>> "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."
>>
>> I wouldn't be surprised if other architectures had similar constraints.
>
> I don't understand how this is an argument against Boaz's approach.  If
> non-temporal stores are essentially broken, they are broken for both the
> kernel use case and for the userspace use case, and (if we want to support
> these platforms, which I'm not sure we do) we would need to fall back to
> writes + explicit flushes for both kernel space and userspace.

MAP_PMEM_AWARE only declares self-awareness does not guarantee that
everyone else sharing the mapping is equally aware.  A pmem-aware app
on such an architecture would be free to flush once and use
non-temporal stores going forward, but if the mapping is shared it
needs to flush all the time.  Like I said before it needs to be
all-aware apps in a shared mapping or none, but it's moot because I
think something like MAP_SYNC is semantically much clearer.

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