linux-kernel - Re: [RFC PATCH] mm: support large folio numa balancing

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Message-ID: <ad7c3a3d-58d8-4edd-859e-4ed5a35145dd@redhat.com>
Date:   Wed, 15 Nov 2023 11:47:11 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        akpm@...ux-foundation.org
Cc:     ying.huang@...el.com, wangkefeng.wang@...wei.com,
        willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: support large folio numa balancing

On 15.11.23 11:46, David Hildenbrand wrote:
> On 13.11.23 11:45, Baolin Wang wrote:
>> Currently, the file pages already support large folio, and supporting for
>> anonymous pages is also under discussion[1]. Moreover, the numa balancing
>> code are converted to use a folio by previous thread[2], and the migrate_pages
>> function also already supports the large folio migration.
>>
>> So now I did not see any reason to continue restricting NUMA balancing for
>> large folio.
>>
>> [1] https://lkml.org/lkml/2023/9/29/342
>> [2] https://lore.kernel.org/all/20230921074417.24004-4-wangkefeng.wang@huawei.com/T/#md9d10fe34587229a72801f0d731f7457ab3f4a6e
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> ---
> 
> I'll note that another piece is missing, and I'd be curious how you
> tested your patch set or what I am missing. (no anonymous pages?)
> 
> change_pte_range() contains:
> 
> if (prot_numa) {
> 	...
> 	/* Also skip shared copy-on-write pages */
> 	if (is_cow_mapping(vma->vm_flags) &&
> 	    folio_ref_count(folio) != 1)
> 		continue;
> 
> So we'll never end up mapping an anon PTE-mapped THP prot-none (well, unless a
> single PTE remains) and consequently never trigger NUMA hinting faults.
> 
> Now, that change has some history [1], but the original problem has been
> sorted out in the meantime. But we should consider Linus' original feedback.
> 
> For pte-mapped THP, we might want to do something like the following
> (completely untested):
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 81991102f785..c4e6b9032e40 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -129,7 +129,8 @@ static long change_pte_range(struct mmu_gather *tlb,
>    
>                                   /* Also skip shared copy-on-write pages */
>                                   if (is_cow_mapping(vma->vm_flags) &&
> -                                   folio_ref_count(folio) != 1)
> +                                   (folio_maybe_dma_pinned(folio) ||
> +                                    folio_estimated_sharers(folio) != 1))

Actually, > 1 might be better if the first subpage is not mapped; it's a 
mess.

-- 
Cheers,

David / dhildenb