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@xxxxxxxxx> > > > > 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@xxxxxxxxxxxxx/ [2] https://lore.kernel.org/lkml/20201012195354.GC2046448@xxxxxxxxxxxxxxxxxxxxxxxxx/ [3] https://lore.kernel.org/lkml/20201009213434.GA839@sol.localdomain/ https://lore.kernel.org/lkml/20201013200149.GI3576660@xxxxxxxxxxxxxxxxxx/
