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

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.

Subsequent patches will convert most ~90% of the kmap callers to this new call
leaving about 10% of the existing kmap callers to enable PKS globally.

Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
---
 include/linux/highmem.h | 34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

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/