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

[Ira Weiny]

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().

	https://lore.kernel.org/lkml/YzN+ZYLjK6HI1P1C@ZenIV/

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.

Ira