linux-kernel - Re: [PATCH v2 1/2] mm/vmalloc: Add interfaces to free unmapped page table

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Message-Id: <20180314153835.68e75da3fdc18b27ad0e290c@linux-foundation.org>
Date:   Wed, 14 Mar 2018 15:38:35 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Toshi Kani <toshi.kani@....com>
Cc:     mhocko@...e.com, 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,
        willy@...radead.org, cpandya@...eaurora.org, linux-mm@...ck.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/vmalloc: Add interfaces to free unmapped page
 table

On Wed, 14 Mar 2018 12:01:54 -0600 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.
> x86 still has memory leak.
> 
> The patch changes the ioremap path to free unmapped page table(s) since
> doing so in the unmap path has the following issues:
> 
>  - The iounmap() path is shared with vunmap().  Since vmap() only
>    supports pte mappings, making vunmap() to free a pte page is an
>    overhead for regular vmap users as they do not need a pte page
>    freed up.
>  - Checking if all entries in a pte page are cleared in the unmap path
>    is racy, and serializing this check is expensive.
>  - The unmap path calls free_vmap_area_noflush() to do lazy TLB purges.
>    Clearing a pud/pmd entry before the lazy TLB purges needs extra TLB
>    purge.
> 
> 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.
> 

whoops.

--- a/include/asm-generic/pgtable.h~mm-vmalloc-add-interfaces-to-free-unmapped-page-table-fix
+++ a/include/asm-generic/pgtable.h
@@ -1014,7 +1014,7 @@ static inline int pud_free_pmd_page(pud_
 {
 	return 0;
 }
-static inline int pmd_free_pte_page(pud_t *pmd)
+static inline int pmd_free_pte_page(pmd_t *pmd)
 {
 	return 0;
 }
_