Re: [PATCH resend] kmap_local: don't assume kmap PTEs are linear arrays in memory

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 16 Nov 2021 10:47:37 +0100 Ard Biesheuvel <ardb@kernel.org> wrote:

> The kmap_local conversion broke the ARM architecture, because the new
> code assumes that all PTEs used for creating kmaps form a linear array
> in memory, and uses array indexing to look up the kmap PTE belonging to
> a certain kmap index.
> 
> On ARM, this cannot work, not only because the PTE pages may be
> non-adjacent in memory, but also because ARM/!LPAE interleaves hardware
> entries and extended entries (carrying software-only bits) in a way that
> is not compatible with array indexing.
> 
> Fortunately, this only seems to affect configurations with more than 8
> CPUs, due to the way the per-CPU kmap slots are organized in memory.
> 
> Work around this by permitting an architecture to set a Kconfig symbol
> that signifies that the kmap PTEs do not form a lineary array in memory,
> and so the only way to locate the appropriate one is to walk the page
> tables.

A Fixes: link would be helpful.

kmap_local() was a year ago, so is a -stable backport desired here?