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SubjectRe: [PATCH 1/2] mm/vmalloc: Add interfaces to free unused page table
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.
>
> index 004abf9ebf12..942f4fa341f1 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -702,4 +702,24 @@ int pmd_clear_huge(pmd_t *pmd)
>
> return 0;
> }
> +
> +/**
> + * pud_free_pmd_page - clear pud entry and free pmd page
> + *
> + * Returns 1 on success and 0 on failure (pud not cleared).
> + */
> +int pud_free_pmd_page(pud_t *pud)
> +{
> + return pud_none(*pud);
> +}
> +
> +/**
> + * pmd_free_pte_page - clear pmd entry and free pte page
> + *
> + * Returns 1 on success and 0 on failure (pmd not cleared).
> + */
> +int pmd_free_pte_page(pmd_t *pmd)
> +{
> + return pmd_none(*pmd);
> +}

Are these functions well named? I mean, the comment says "clear pud
entry and free pmd page" but the implementatin does neither of those
things. The name implies that the function frees a pmd_page but the
callsites use the function as a way of querying state.


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