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

[Christophe Leroy <christophe.leroy@c-s.fr>]

Le 07/06/2019 à 05:56, Nicholas Piggin a écrit :
> 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.
> 
> Fixes: da7ad366b497 ("powerpc/mm/book3s: Update pmd_present to look at _PAGE_PRESENT bit")
> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
> Suggested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin@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
> +

Could get rid of that #ifdef by adding the following in book3s32 and 
nohash pgtable.h:

static inline bool pmd_is_serializing()  { return false; }

Christophe

>   	if (pmd_trans_huge(pmd) || pmd_devmap(pmd)) {
>   		if (is_thp)
>   			*is_thp = true;
>