linux-kernel - Re: [PATCH -next v2 09/19] mm: mprotect: use a folio in change_pte

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Date:   Sat, 14 Oct 2023 11:11:54 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <ying.huang@...el.com>,
        <david@...hat.com>, Zi Yan <ziy@...dia.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH -next v2 09/19] mm: mprotect: use a folio in
 change_pte_range()



On 2023/10/13 23:13, Matthew Wilcox wrote:
> On Fri, Oct 13, 2023 at 04:55:53PM +0800, Kefeng Wang wrote:
>> Use a folio in change_pte_range() to save three compound_head() calls.
> 
> Yes, but here we have a change of behaviour, which should be argued
> is desirable.  Before if a partial THP was mapped, or a fs large
> folio, we would do this to individual pages.  Now we're doing it to the
> entire folio.  Is that desirable?  I don't have the background to argue
> either way.

The Huang's replay in v1[1] already mentioned this, we only use 
last_cpupid from head page, and large folio won't be handled from
do_numa_page(), and if large folio numa balancing is supported,
we could migrate the entire large folio mapped only one process,
or maybe split the large folio mapped multi-processes, and when
split it, we will copy the last_cpupid from head to the tail page.
Anyway, I think this change or the wp_page_reuse() won't break
current numa balancing.

Thanks.


[1]https://lore.kernel.org/linux-mm/874jixhfeu.fsf@yhuang6-desk2.ccr.corp.intel.com/
> 
>> @@ -157,7 +159,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   					continue;
>>   				if (sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING &&
>>   				    !toptier)
>> -					xchg_page_access_time(page,
>> +					folio_xchg_access_time(folio,
>>   						jiffies_to_msecs(jiffies));
>>   			}
>