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

From: Ard Biesheuvel <ardb@kernel.org>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	 Linus Walleij <linus.walleij@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>,
	 Linux ARM <linux-arm-kernel@lists.infradead.org>,
	 Quanyang Wang <quanyang.wang@windriver.com>
Subject: Re: [PATCH] kmap_local: don't assume kmap PTEs are linear arrays in memory
Date: Mon, 15 Nov 2021 08:54:33 +0100	[thread overview]
Message-ID: <CAMj1kXFQ9VZo9Npys7BbjmvTP8Bfc_WaEQNaHnETPj-XHV2S7g@mail.gmail.com> (raw)
In-Reply-To: <YYpfxHNgWBDq/iiv@shell.armlinux.org.uk>

On Tue, 9 Nov 2021 at 12:47, Russell King (Oracle)
<linux@armlinux.org.uk> wrote:
>
> On Fri, Nov 05, 2021 at 04:01:20PM +0100, Ard Biesheuvel wrote:
> > On Fri, 5 Nov 2021 at 15:50, Russell King (Oracle)
> > <linux@armlinux.org.uk> wrote:
> > >
> > > On Fri, Nov 05, 2021 at 03:21:00PM +0100, Ard Biesheuvel wrote:
> > > > On Tue, 26 Oct 2021 at 15:13, 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.
> > > > >
> > > > > Reported-by: Quanyang Wang <quanyang.wang@windriver.com>
> > > > > Signed-off-by: Ard Biesheuvel <ardb@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.
> > >
> >
> > I was hoping Thomas would respond, given that he introduced the
> > problem in the first place, and can either carry the fix himself, or
> > tell us whether he wants it fixed in a different way.
>
> We're two weeks on, and Thomas hasn't responded - not on IRC nor via
> email. Can we get akpm to pick the patch up? (Original patch message
> attached.)
>
> I think I have one fix ("fix early early_iounmap()") that needs to go
> to Linus, so alternatively I could send it with that... if someone
> wants to drop it in the patch system.
>

Submitted as 9157/1

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