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

[Russell King (Oracle)]

On Fri, Nov 05, 2021 at 03:21:00PM +0100, Ard Biesheuvel wrote:
> On Tue, 26 Oct 2021 at 15:13, Ard Biesheuvel <ardb at 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.
> >
> > Reported-by: Quanyang Wang <quanyang.wang at windriver.com>
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> > ---
> 
> Ping? Can we get this fixed please?

Who are you expecting to apply it? It seems to be touching only core
code, so I don't think it's up to me - not without some kind of review
from mm guys.

-- 
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