Re: [RFC REBASED 5/5] powerpc/mm/slice: use the dynamic high slice size to limit bitmap operations

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

On 02/28/2018 12:23 PM, Nicholas Piggin wrote:
> On Tue, 27 Feb 2018 18:11:07 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
> 
>> Nicholas Piggin <npiggin@gmail.com> writes:
>>
>>> On Tue, 27 Feb 2018 14:31:07 +0530
>>> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
>>>   
>>>> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>>>    
>>>>> The number of high slices a process might use now depends on its
>>>>> address space size, and what allocation address it has requested.
>>>>>
>>>>> This patch uses that limit throughout call chains where possible,
>>>>> rather than use the fixed SLICE_NUM_HIGH for bitmap operations.
>>>>> This saves some cost for processes that don't use very large address
>>>>> spaces.
>>>>
>>>> I haven't really looked at the final code. One of the issue we had was
>>>> with the below scenario.
>>>>
>>>> mmap(addr, len) where addr < 128TB and addr+len > 128TB  We want to make
>>>> sure we build the mask such that we don't find the addr available.
>>>
>>> We should run it through the mmap regression tests. I *think* we moved
>>> all of that logic from the slice code to get_ummapped_area before going
>>> in to slices. I may have missed something though, it would be good to
>>> have more eyes on it.
>>>   
>>
>> mmap(-1,...) failed with the test. Something like below fix it
>>
>> @@ -756,7 +770,7 @@ void slice_set_user_psize(struct mm_struct *mm, unsigned int psize)
>>          mm->context.low_slices_psize = lpsizes;
>>   
>>          hpsizes = mm->context.high_slices_psize;
>> -       high_slices = GET_HIGH_SLICE_INDEX(mm->context.slb_addr_limit);
>> +       high_slices = SLICE_NUM_HIGH;
>>          for (i = 0; i < high_slices; i++) {
>>                  mask_index = i & 0x1;
>>                  index = i >> 1;
>>
>> I guess for everything in the mm_context_t, we should compute it till
>> SLICE_NUM_HIGH. The reason for failure was, even though we recompute the
>> slice mask cached in mm_context on slb_addr_limit, it was still derived
>> from the high_slices_psizes which was computed with lower value.
> 
> Okay thanks for catching that Aneesh. I guess that's a slow path so it
> should be okay. Christophe if you're taking care of the series can you
> fold it in? Otherwise I'll do that after yours gets merged.
> 

should we also compute the mm_context_t.slice_mask using SLICE_NUM_HIGH 
and skip the recalc_slice_mask_cache when we change the addr limit?

-aneesh