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

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Message-ID: <05dbc7b8-2e4a-4762-a6a6-278985d89928@redhat.com>
Date:   Wed, 15 Nov 2023 11:46:02 +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 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))
                                         continue;
  

Another note about the possible imprecision that might or might not
be tolerable ;)


[1] https://bugzilla.kernel.org/show_bug.cgi?id=215616

-- 
Cheers,

David / dhildenb