Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends

[Linus Torvalds]

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