Re: [PATCH] x86, pmem: fix broken __copy_user_nocache cache-bypass assumptions

[Kani, Toshimitsu]

On Thu, 2017-04-06 at 13:59 -0700, Dan Williams wrote:
> Before we rework the "pmem api" to stop abusing __copy_user_nocache()
> for memcpy_to_pmem() we need to fix cases where we may strand dirty
> data in the cpu cache. The problem occurs when copy_from_iter_pmem()
> is used for arbitrary data transfers from userspace. There is no
> guarantee that these transfers, performed by dax_iomap_actor(), will
> have aligned destinations or aligned transfer lengths. Backstop the
> usage __copy_user_nocache() with explicit cache management in these
> unaligned cases.
> 
> Yes, copy_from_iter_pmem() is now too big for an inline, but
> addressing that is saved for a later patch that moves the entirety of
> the "pmem api" into the pmem driver directly.

The change looks good to me.  Should we also avoid cache flushing in
the case of size=4B & dest aligned by 4B?

Thanks,
-Toshi