Re: [PATCH v2 1/3] introduce memcpy_nocache()

From: Al Viro
Date: Wed Dec 28 2016 - 18:43:38 EST

On Tue, Nov 01, 2016 at 04:25:12PM +0200, Boaz Harrosh wrote:

> >> What about memcpy_to_pmem() in linux/pmem.h it already has all the arch switches.
> >>
> >> Feels bad to add yet just another arch switch over __copy_user_nocache
> >>
> >> Just feels like too many things that do the same thing. Sigh
> >
> > I agree that this looks like a nicer path.
> >
> > I had considered adjusting copy_from_iter_nocache() to use memcpy_to_pmem(),
> > but lib/iov_iter.c doesn't currently #include linux/pmem.h. Would it be
> > acceptable to add it? Also, I wasn't sure if memcpy_to_pmem() would always
> > mean exactly "memcpy nocache".
> >
> I think this is the way to go. In my opinion there is no reason why not to include
> pmem.h into lib/iov_iter.c.
> And I think memcpy_to_pmem() would always be the fastest arch way to bypass cache
> so it should be safe to use this for all cases. It is so in the arches that support
> this now, and I cannot imagine a theoretical arch that would differ. But let the
> specific arch people holler if this steps on their tows, later when they care about
> this at all.

First of all, if it's the fastest arch way to bypass cache, why the hell
is it sitting in pmem-related areas?

More to the point, x86 implementation of that thing is tied to uaccess API
for no damn reason whatsoever. Let's add a real memcpy_nocache() and
be done with that. I mean, this
if (WARN(rem, "%s: fault copying %p <- %p unwritten: %d\n",
__func__, dst, src, rem))
is *screaming* "API misused here". And let's stay away from the STAC -
it's pointless for kernel-to-kernel copies.

BTW, your "it's iovec, only non-temporal stores there" logics in
arch_copy_from_iter_pmem() is simply wrong - for one thing, unaligned
copies will have parts done via normal stores, for another 32bit will
_not_ go for non-caching codepath for short copies. What semantics do
we really need there?