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

[Nicholas Piggin <npiggin@gmail.com>]

Christophe Leroy's on June 7, 2019 3:35 pm:
> 
> 
> 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; }

I don't mind either way. If it's an isolated case like this, sometimes 
I'm against polluting the sub arch code with it.

It's up to you I can change that if you prefer.

Thanks,
Nick