On Tue, 16 Nov 2021 at 23:12, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 16 Nov 2021 10:47:37 +0100 Ard Biesheuvel <ardb@xxxxxxxxxx> 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")
Cc: <stable@xxxxxxxxxxxxxxx>
