[LSF/MM/BPF] Whither Highmem?

[Matthew Wilcox <willy@infradead.org>]

I see there's a couple of spots on the schedule open, so here's something
fun we could talk about.

Highmem was originally introduced to support PAE36 (up to 64GB) on x86
in the late 90s.  It's since been used to support a similar extension
on ARM (maybe other 32-bit architectures?)

Things have changed a bit since then.  There aren't a lot of systems
left which have more than 4GB of memory _and_ are incapable of running a
64-bit kernel.  We certainly don't see 64GB systems; maybe 8GB systems,
but 64-bit registers are cheap and if you're willing to solder 8GB of
RAM to the board, you're probably willing to splurge on a 64-bit ALU.

The objection might be raised that kmap_local() is cheap, and it is.
But when we have multi-page folios, particularly for filesystem metadata,
it gets awkward to use kmap because it only supports individual pages;
you can't kmap an entire folio.

So I'd like to explore removing support for keeping filesystem metadata
and page tables in highmem.  Anon memory and file memory should probably
remain supported in highmem.

Interested?