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

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Tue, Nov 16, 2021 at 10:47:37AM +0100, Ard Biesheuvel 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.
> 
> Link: https://lore.kernel.org/linux-arm-kernel/20211026131249.3731275-1-ardb@kernel.org/
> Reported-by: Quanyang Wang <quanyang.wang@windriver.com>
> Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Russell King <linux@armlinux.org.uk>

Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Thanks.

-- 
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