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

[Raghavendra K T <rkodsara@amd.com>]

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>