Re: [PATCH RFC PKS/PMEM 05/58] kmap: Introduce k[un]map_thread

[Ira Weiny <ira.weiny@intel.com>]

On Tue, Nov 10, 2020 at 02:13:56AM +0100, Thomas Gleixner wrote:
> Ira,
> 
> On Fri, Oct 09 2020 at 12:49, ira weiny wrote:
> > From: Ira Weiny <ira.weiny@intel.com>
> >
> > To correctly support the semantics of kmap() with Kernel protection keys
> > (PKS), kmap() may be required to set the protections on multiple
> > processors (globally).  Enabling PKS globally can be very expensive
> > depending on the requested operation.  Furthermore, enabling a domain
> > globally reduces the protection afforded by PKS.
> >
> > Most kmap() (Aprox 209 of 229) callers use the map within a single thread and
> > have no need for the protection domain to be enabled globally.  However, the
> > remaining callers do not follow this pattern and, as best I can tell, expect
> > the mapping to be 'global' and available to any thread who may access the
> > mapping.[1]
> >
> > We don't anticipate global mappings to pmem, however in general there is a
> > danger in changing the semantics of kmap().  Effectively, this would cause an
> > unresolved page fault with little to no information about why the failure
> > occurred.
> >
> > To resolve this a number of options were considered.
> >
> > 1) Attempt to change all the thread local kmap() calls to kmap_atomic()[2]
> > 2) Introduce a flags parameter to kmap() to indicate if the mapping should be
> >    global or not
> > 3) Change ~20 call sites to 'kmap_global()' to indicate that they require a
> >    global enablement of the pages.
> > 4) Change ~209 call sites to 'kmap_thread()' to indicate that the mapping is to
> >    be used within that thread of execution only
> >
> > Option 1 is simply not feasible.  Option 2 would require all of the call sites
> > of kmap() to change.  Option 3 seems like a good minimal change but there is a
> > danger that new code may miss the semantic change of kmap() and not get the
> > behavior the developer intended.  Therefore, #4 was chosen.
> 
> There is Option #5:

There is now yes.  :-D

> 
> Convert the thread local kmap() invocations to the proposed kmap_local()
> interface which is coming along [1].

I've been trying to follow that thread.

> 
> That solves a couple of issues:
> 
>  1) It relieves the current kmap_atomic() usage sites from the implict
>     pagefault/preempt disable semantics which apply even when
>     CONFIG_HIGHMEM is disabled. kmap_local() still can be invoked from
>     atomic context.
> 
>  2) Due to #1 it allows to replace the conditional usage of kmap() and
>     kmap_atomic() for purely thread local mappings.
> 
>  3) It puts the burden on the HIGHMEM inflicted systems
> 
>  4) It is actually more efficient for most of the pure thread local use
>     cases on HIGHMEM inflicted systems because it avoids the overhead of
>     the global lock and the potential kmap slot exhaustion. A potential
>     preemption will be more expensive, but that's not really the case we
>     want to optimize for.
> 
>  5) It solves the RT issue vs. kmap_atomic()
> 
> So instead of creating yet another variety of kmap() which is just
> scratching the particular PKRS itch, can we please consolidate all of
> that on the wider reaching kmap_local() approach?

Yes I agree.  We absolutely don't want more kmap*() calls and I was hoping to
dovetail into your kmap_local() work.[2]

I've pivoted away from this work a bit to clean up all the
kmap()/memcpy*()/kunmaps() as discussed elsewhere in the thread first.[3]  I
was hoping your work would land and then I could s/kmap_thread()/kmap_local()/
on all of these patches.

Also, we can convert the new memcpy_*_page() calls to kmap_local() as well.
[For now my patch just uses kmap_atomic().]

I've not looked at all of the patches in your latest version.  Have you
included converting any of the kmap() call sites?  I thought you were more
focused on converting the kmap_atomic() to kmap_local()?

Ira

> 
> Thanks,
> 
>         tglx
>      
> [1] https://lore.kernel.org/lkml/20201103092712.714480842@linutronix.de/

[2] https://lore.kernel.org/lkml/20201012195354.GC2046448@iweiny-DESK2.sc.intel.com/
[3] https://lore.kernel.org/lkml/20201009213434.GA839@sol.localdomain/
    https://lore.kernel.org/lkml/20201013200149.GI3576660@ZenIV.linux.org.uk/