Re: [PATCH v2 7/7] pmem, dax: have direct_access use __pmem annotation

From: Dan Williams
Date: Sat Aug 15 2015 - 11:44:46 EST

On Sat, Aug 15, 2015 at 2:19 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> On Thu, Aug 13, 2015 at 10:51:11AM -0600, Ross Zwisler wrote:
>> Update the annotation for the kaddr pointer returned by direct_access()
>> so that it is a __pmem pointer. This is consistent with the PMEM driver
>> and with how this direct_access() pointer is used in the DAX code.
> IFF we stick to the __pmem annotations this looks good.
> That beeing said I start to really dislike them. We don't special
> accesors to read/write from pmem, we just need to explicitly commit
> it if we want to make it persistent. So I really don't see the need
> to treat it special and require all the force casts to and from the
> attribute.

I'm not going to put up much of a fight if it's really getting in the way....

That said, while we don't need special accessors we do need guarantees
that anything that has written to a persistent memory address has done
so in a way that wmb_pmem() is able to flush it. It's more of a "I've
audited this code path for wmb_pmem() compatibility so use this api to
write to pmem."

Perhaps a better way to statically check for missed flushes might be
to have acquire_pmem_for_write() + release() annotations and the final
release does a wmb_pmem(), but as far as I can tell the sparse
acquire/release annotations don't stack.
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