On Fri, Oct 30, 2020 at 3:26 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > While at it I might have a look at that debug request from Willy in the > other end of this thread. Any comment on that? > > https://lore.kernel.org/r/87k0v7mrrd.fsf@xxxxxxxxxxxxxxxxxxxxxxx I do think that it would be nice to have a debug mode, particularly since over the last few years we've really lost a lot of HIGHMEM coverage (to the point that I've wondered how worthwhile it really is to support at all any more - I think it's Arnd who argued that it's mainly some embedded ARM variants that will want it for the forseeable future). So I'm honestly somewhat torn. I think HIGHMEM is dying, and yes that argues for "non-HIGHMEM had better have some debug coverage", but at the same time I think it doesn't even really matter any more. At some point those embedded ARM platforms just aren't even interesting - they might as well use older kernels if they are the only thing really arguing for HIGHMEM at all. This is one reason why I'd like the new kmap_local() to be a no-op, and I'd prefer for it to have no other side effects - because I want to be ready to remove it entirely some day. And if we end up having some transition where people start rewriting "kmap_atomic()" to be "kmap_local() + explicit preemption disable", then I think that would be a good step on that whole "kmap will eventually go away" path. But I do *not* believe that we need to add _so_ much debug support that we'd catch Willy's "more than one page" case. And I absolutely do not believe for a second that we should start caring about compound pages. NO. kmap() is almost dead already, we're not making it worse. To me, your patch series has two big advantages: - more common code - kmap_local() becomes more of a no-op and the last thing we want is to expand on kmap. Linus
