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SubjectRe: [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@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.
>
> 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;
}
_
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