linux-kernel - Re: [PATCH 1/2] mm/vmalloc: Add interfaces to free unused page table

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Message-ID: <20180308080847.dvwd3w6wuhwsg3qo@gmail.com>
Date:   Thu, 8 Mar 2018 09:08:47 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Toshi Kani <toshi.kani@....com>
Cc:     mhocko@...e.com, akpm@...ux-foundation.org, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, bp@...e.de,
        catalin.marinas@....com, guohanjun@...wei.com, will.deacon@....com,
        wxf.wang@...ilicon.com, linux-mm@...ck.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2] mm/vmalloc: Add interfaces to free unused page table


* Toshi Kani <toshi.kani@....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.

Where does x86 iounmap() do that?

> 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.

At minimum the ordering of the patches is very confusing: why don't you introduce 
the new methods in patch #1, and then use them in patch #2?

Also please double check the coding style of your patches, there's a number of 
obvious problems of outright bad patterns and also cases where you clearly don't 
try to follow the (correct) style of existing code.

Thanks,

	Ingo