Re: [PATCH V2] highmem: Fix kmap_to_page() for kmap_local_page() addresses

From: Ira Weiny
Date: Fri Oct 07 2022 - 11:46:05 EST

On Thu, Oct 06, 2022 at 01:33:31PM -0700, Andrew Morton wrote:
> On Wed, 5 Oct 2022 21:05:55 -0700 ira.weiny@xxxxxxxxx wrote:
> > kmap_to_page() is used to get the page for a virtual address which may
> > be kmap'ed. Unfortunately, kmap_local_page() stores mappings in a
> > thread local array separate from kmap(). These mappings were not
> > checked by the call.
> >
> > Check the kmap_local_page() mappings and return the page if found.
> >
> > Because it is intended to remove kmap_to_page() add a warn on once to
> > the kmap checks to flag potential issues early.
> What were the user-visible runtime effects of this?

No one actually hit a bug with this because AFAIK only one kmap() call has been
converted to kmap_local_page() which then eventually calls kmap_to_page().

However that has already been fixed by Al in that thread.

> Are we able to identify a Fixes:?

I suppose this could be added as a Fixes: to the original patch introducing
kmap_local_page()? But one could argue that kmap_to_page() was only intended
to support kmap() addresses because it does not work with kmap_atomic()
addresses either.

I'm proposing this as a stop gap to ensure that work can continue on converting
kmap() uses to kmap_local_page() without fear of causing breakage while
simultaneously we evaluate and hopefully remove kmap_to_page() as well.