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

[David Hildenbrand]

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@xxxxxxxxxx/T/#md9d10fe34587229a72801f0d731f7457ab3f4a6e
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
>
> 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