Re: [RFC PATCH 1/2] mm/mmu_gather: Invalidate TLB correctly on batch allocation failure and flush

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 12/17/19 2:39 PM, Peter Zijlstra wrote:
> On Tue, Dec 17, 2019 at 12:47:12PM +0530, Aneesh Kumar K.V wrote:
>> Architectures for which we have hardware walkers of Linux page table should
>> flush TLB on mmu gather batch allocation failures and batch flush. Some
>> architectures like POWER supports multiple translation modes (hash and radix)
>> and in the case of POWER only radix translation mode needs the above TLBI.
>> This is because for hash translation mode kernel wants to avoid this extra
>> flush since there are no hardware walkers of linux page table. With radix
>> translation, the hardware also walks linux page table and with that, kernel
>> needs to make sure to TLB invalidate page walk cache before page table pages are
>> freed.
> 
>> Based on changes from Peter Zijlstra <peterz@infradead.org>
> 
> AFAICT it is all my patch ;-)

Yes. I moved the changes you had to upstream. I can update the From: in 
the next version if you are ok with that?

> 
> Anyway, this commit:
> 
>> More details in
>> commit: d86564a2f085 ("mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE")
> 
> states that you do an explicit invalidate in __p*_free_tlb(), which, if
> I'm not mistaken is still there:
> 
>    arch/powerpc/include/asm/nohash/pgalloc.h:      tlb_flush_pgtable(tlb, address);
> 

nohash is not really radix. So we still do the tlb flush from the 
pte_free_tlb for nohash and for PPC-radix, we let tlb_table_invalidate 
to flush that.


> Or am I reading this wrong? I'm thinking you can remove that now.
> 
>> diff --git a/arch/powerpc/include/asm/tlb.h b/arch/powerpc/include/asm/tlb.h
>> index b2c0be93929d..feea1a09bbce 100644
>> --- a/arch/powerpc/include/asm/tlb.h
>> +++ b/arch/powerpc/include/asm/tlb.h
>> @@ -27,6 +27,10 @@
>>   #define tlb_flush tlb_flush
>>   extern void tlb_flush(struct mmu_gather *tlb);
>>   
>> +#ifdef CONFIG_HAVE_RCU_TABLE_FREE
> /*
>   * PPC-Hash does not use the linux page-tables, so we can avoid
>   * the TLBI for page-table freeing, PPC-Radix otoh does use the
>   * page-tables and needs the TLBI.
>   */
>> +#define tlb_needs_table_invalidate()	radix_enabled()
>> +#endif
> 
> Also, are you really sure about the !SMP case? Esp. on Radix I'm
> thinking that the PWC (page-walk-cache) can give trouble even on UP,
> when we get preempted in the middle of mmu_gather. Hmm?
> 

Yes, looking at !SMP I guess we do have issue there. we do free the 
pagetable pages directly in __p*_free_tlb() with the current code. That 
will definitely not work. Are you suggesting we enable 
HAVE_RCU_TABLE_FREE even for !SMP?

>>   /* Get the generic bits... */
>>   #include <asm-generic/tlb.h>
> 
> 

-aneesh