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

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Message-ID: <871pnfgrr8.fsf@oracle.com>
Date: Mon, 06 Oct 2025 23:17:31 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Raghavendra K T <rkodsara@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Ankur Arora
 <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org, david@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        mjguzik@...il.com, luto@...nel.org, peterz@...radead.org,
        acme@...nel.org, namhyung@...nel.org, tglx@...utronix.de,
        willy@...radead.org, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v7 13/16] mm: memory: support clearing page ranges


Raghavendra K T <rkodsara@....com> writes:

> On 9/18/2025 3:14 AM, Andrew Morton wrote:
>> On Wed, 17 Sep 2025 08:24:15 -0700 Ankur Arora <ankur.a.arora@...cle.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@....com>

Thanks Raghu.

--
ankur