Re: [PATCH v1 1/3] mm: Allow deferred splitting of arbitrary large anon folios

[David Hildenbrand]

On 17.07.23 17:41, Ryan Roberts wrote:
> On 17/07/2023 16:30, Matthew Wilcox wrote:
> > On Mon, Jul 17, 2023 at 03:31:08PM +0100, Ryan Roberts wrote:
> > > In preparation for the introduction of large folios for anonymous
> > > memory, we would like to be able to split them when they have unmapped
> > > subpages, in order to free those unused pages under memory pressure. So
> > > remove the artificial requirement that the large folio needed to be at
> > > least PMD-sized.
> > >
> > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > > Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> > > Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> >
> > Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
>
> Thanks!
>
> > >   		 */
> > > -		if (folio_test_pmd_mappable(folio) && folio_test_anon(folio))
> > > +		if (folio_test_large(folio) && folio_test_anon(folio))
> > >   			if (!compound || nr < nr_pmdmapped)
> > >   				deferred_split_folio(folio);
> >
> > I wonder if it's worth introducing a folio_test_deferred_split() (better
> > naming appreciated ...) to allow us to allocate order-1 folios and not
> > do horrible things.  Maybe it's not worth supporting order-1 folios;
> > we're always better off going to order-2 immediately.  Just thinking.
>
> There is more than just _deferred_list in the 3rd page; you also have _flags_2a
> and _head_2a. I guess you know much better than me what they store. But I'm
> guessing its harder than jsut not splitting an order-1 page?
>
> With the direction of large anon folios (_not_ retrying with every order down to
> 0), I'm not sure what the use case would be for order-1 anyway?

Just noting that we might need some struct-page space for better 
mapcount/shared tracking, which might get hard for order-1 pages.

--
Cheers,

David / dhildenb