[PATCH] arm64: mm: fix pmd_write CoW brokenness

[marc.zyngier@arm.com (Marc Zyngier)]

Hi Will,

On 28/05/14 18:07, Will Deacon wrote:
> Commit 9c7e535fcc17 ("arm64: mm: Route pmd thp functions through pte
> equivalents") changed the pmd manipulator and accessor functions to
> convert the target pmd to a pte, process it with the pte functions, then
> convert it back. Along the way, we gained support for PTE_WRITE, however
> this is completely ignored by set_pmd_at, and so we fail to set the
> PMD_SECT_RDONLY for PMDs, resulting in all sorts of lovely failures (like
> CoW not working).
> 
> Partially reverting the offending commit (by making use of
> PMD_SECT_RDONLY explicitly for pmd_{write,wrprotect,mkwrite} functions)
> leads to further issues because pmd_write can then return potentially
> incorrect values for page table entries marked as RDONLY, leading to
> BUG_ON(pmd_write(entry)) tripping under some THP workloads.
> 
> This patch fixes the issue by routing set_pmd_at through set_pte_at,
> which correctly takes the PTE_WRITE flag into account. Given that
> THP mappings are always anonymous, the additional cache-flushing code
> in __sync_icache_dcache won't impose any significant overhead as the
> flush will be skipped.
> 
> Cc: Steve Capper <steve.capper@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
> 
> Whilst this is a fairly scary change at this point in the cycle, I can't get
> through an LTP run without it. Furthermore, spurious CoW failures for tasks
> that happen to get transparent hugepages are a pretty significant regression.
> 
>  arch/arm64/include/asm/pgtable.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 90c811f05a2e..20785f9da95c 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -266,7 +266,7 @@ static inline pmd_t pte_pmd(pte_t pte)
>  
>  #define pmd_page(pmd)           pfn_to_page(__phys_to_pfn(pmd_val(pmd) & PHYS_MASK))
>  
> -#define set_pmd_at(mm, addr, pmdp, pmd)	set_pmd(pmdp, pmd)
> +#define set_pmd_at(mm, addr, pmdp, pmd)	set_pte_at(mm, addr, pmdp, pmd_pte(pmd))
>  
>  static inline int has_transparent_hugepage(void)
>  {
> 

I managed to reliably reproduce the failure on my favourite arm64 board
running a KVM guest with THP enabled. Adding this patch to the guest
kernel made it behave correctly.

So FWIW:
Tested-by: Marc Zyngier <marc.zyngier@arm.com>

	M.
-- 
Jazz is not dead. It just smells funny...