[PATCH 2/2] powerpc/64s: __find_linux_pte synchronization vs pmdp_invalidate

[Aneesh Kumar K.V]

Nicholas Piggin <npiggin at gmail.com> writes:

> The change to pmdp_invalidate to mark the pmd with _PAGE_INVALID broke
> the synchronisation against lock free lookups, __find_linux_pte's
> pmd_none check no longer returns true for such cases.
>
> Fix this by adding a check for this condition as well.
>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>

> Fixes: da7ad366b497 ("powerpc/mm/book3s: Update pmd_present to look at _PAGE_PRESENT bit")
> Cc: Christophe Leroy <christophe.leroy at c-s.fr>
> Suggested-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> ---
>  arch/powerpc/mm/pgtable.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
> index db4a6253df92..533fc6fa6726 100644
> --- a/arch/powerpc/mm/pgtable.c
> +++ b/arch/powerpc/mm/pgtable.c
> @@ -372,13 +372,25 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
>  	pdshift = PMD_SHIFT;
>  	pmdp = pmd_offset(&pud, ea);
>  	pmd  = READ_ONCE(*pmdp);
> +
>  	/*
> -	 * A hugepage collapse is captured by pmd_none, because
> -	 * it mark the pmd none and do a hpte invalidate.
> +	 * A hugepage collapse is captured by this condition, see
> +	 * pmdp_collapse_flush.
>  	 */
>  	if (pmd_none(pmd))
>  		return NULL;
>  
> +#ifdef CONFIG_PPC_BOOK3S_64
> +	/*
> +	 * A hugepage split is captured by this condition, see
> +	 * pmdp_invalidate.
> +	 *
> +	 * Huge page modification can be caught here too.
> +	 */
> +	if (pmd_is_serializing(pmd))
> +		return NULL;
> +#endif
> +
>  	if (pmd_trans_huge(pmd) || pmd_devmap(pmd)) {
>  		if (is_thp)
>  			*is_thp = true;
> -- 
> 2.20.1