Re: [PATCH v7 13/16] mm: memory: support clearing page ranges

[Raghavendra K T <raghavendra.kt@amd.com>]

On 9/23/2025 2:06 PM, Raghavendra K T wrote:
> 
> 
> On 9/18/2025 3:14 AM, Andrew Morton wrote:
>> On Wed, 17 Sep 2025 08:24:15 -0700 Ankur Arora 
>> <ankur.a.arora@oracle.com> wrote:
>>
>>> Change folio_zero_user() to clear contiguous page ranges instead of
>>> clearing using the current page-at-a-time approach. Exposing the largest
>>> feasible length can be useful in enabling processors to optimize based
>>> on extent.
>>
>> This patch is something which MM developers might care to take a closer
>> look at.
>>
>>> However, clearing in large chunks can have two problems:
>>>
>>>   - cache locality when clearing small folios (< MAX_ORDER_NR_PAGES)
>>>     (larger folios don't have any expectation of cache locality).
>>>
>>>   - preemption latency when clearing large folios.
>>>
>>> Handle the first by splitting the clearing in three parts: the
>>> faulting page and its immediate locality, its left and right
>>> regions; with the local neighbourhood cleared last.
>>
>> Has this optimization been shown to be beneficial?
>>
>> If so, are you able to share some measurements?
>>
>> If not, maybe it should be removed?
>>
> 
> I reverted the effect of this patch by hard coding
> 
> #define PAGE_CONTIG_NR 1
> 
> I see that benefit for voluntary kernel is lost without this change
> 
> (for rep stosb)
> 
> with PAGE_CONTIG_NR equivalent to 8MB
> 
> Preempt mode: voluntary
> 
> # Running 'mem/mmap' benchmark:
> # function 'demand' (Demand loaded mmap())
> # Copying 64GB bytes ...
> 
>        34.533414 GB/sec
> 
> 
> with PAGE_CONTIG_NR equivalent to 4KB
> 
> # Running 'mem/mmap' benchmark:
> # function 'demand' (Demand loaded mmap())
> # Copying 64GB bytes ...
> 
>        20.766059 GB/sec
> 
> For now (barring David's recommendations),
> feel free to add
> 
> Reviewed-by: Raghavendra K T <raghavendra.kt@amd.com>
> 
> 

My reply was more towards the benefits of clearing multiple pages
for non-preempt and voluntary kernel, than the effect of clearing
neighborhood range at the end.

Sorry if that was confusing :)