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

[Andrew Morton]

On Wed,  7 Mar 2018 11:32:26 -0700 Toshi Kani <toshi.kani at 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.