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Date:   Wed, 7 Mar 2018 14:55:44 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Toshi Kani <toshi.kani@hpe.com>
Cc:     mhocko@suse.com, tglx@linutronix.de, mingo@redhat.com,
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        linux-mm@kvack.org, x86@kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm/vmalloc: Add interfaces to free unused page
 table
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On Wed,  7 Mar 2018 11:32:26 -0700 Toshi Kani <toshi.kani@hpe.com> wrote:

> On architectures with CONFIG_HAVE_ARCH_HUGE_VMAP set, ioremap()
> may create pud/pmd mappings.  Kernel panic was observed on arm64
> systems with Cortex-A75 in the following steps as described by
> Hanjun Guo.
> 
> 1. ioremap a 4K size, valid page table will build,
> 2. iounmap it, pte0 will set to 0;
> 3. ioremap the same address with 2M size, pgd/pmd is unchanged,
>    then set the a new value for pmd;
> 4. pte0 is leaked;
> 5. CPU may meet exception because the old pmd is still in TLB,
>    which will lead to kernel panic.
> 
> This panic is not reproducible on x86.  INVLPG, called from iounmap,
> purges all levels of entries associated with purged address on x86.
> x86 still has memory leak.
> 
> Add two interfaces, pud_free_pmd_page() and pmd_free_pte_page(),
> which clear a given pud/pmd entry and free up a page for the lower
> level entries.
> 
> This patch implements their stub functions on x86 and arm64, which
> work as workaround.

Also, as this fixes an arm64 kernel panic, should we be using cc:stable?