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

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jul 17, 2023 at 05:43:40PM +0200, David Hildenbrand wrote:
> 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@arm.com>
> > > > Reviewed-by: Yu Zhao <yuzhao@google.com>
> > > > Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
> > > 
> > > Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > 
> > 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?

Those are page->flags and page->compound_head for the third page in
the folio.  They don't really need a name; nothing refers to them,
but it's important that space not be reused ;-)

This is slightly different from _flags_1; we do have some flags which
reuse the bits (they're labelled as PF_SECOND).  Right now, it's only
PF_has_hwpoisoned, but we used to have PF_double_map.  Others may arise.

> > 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.

My assumption had been that we'd be able to reuse the _entire_mapcount
and _nr_pages_mapped fields and not spill into the third page, but the
third page is definitely available today if we want it.  I'm fine with
disallowing order-1 anon/file folios forever.