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

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