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

[Ard Biesheuvel <ardb@kernel.org>]

On Tue, 16 Nov 2021 at 23:13, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Tue, 16 Nov 2021 at 23:12, Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > 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?
>
> Fixes: 1834eb1a6027 ("kmap_local: don't assume kmap PTEs are linear
> arrays in memory")

Oops that should be

Fixes: 2a15ba82fa6c ("ARM: highmem: Switch to generic kmap atomic")


> Cc: <stable@vger.kernel.org>