Re: [PATCH 7/9] mm: Add deferred_list page flag

[Matthew Wilcox <willy@infradead.org>]

On Tue, Aug 15, 2023 at 06:40:55PM +0200, David Hildenbrand wrote:
> On 15.08.23 17:32, Matthew Wilcox wrote:
> > On Tue, Aug 15, 2023 at 09:54:36AM +0200, David Hildenbrand wrote:
> > > On 15.08.23 05:26, Matthew Wilcox (Oracle) wrote:
> > > > Stored in the first tail page's flags, this flag replaces the destructor.
> > > > That removes the last of the destructors, so remove all references to
> > > > folio_dtor and compound_dtor.
> > > > 
> > > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > > ---
> > > 
> > > [...]
> > > 
> > > > +	/* Has a deferred list (may be empty).  First tail page. */
> > > > +	PG_deferred_list = PG_reclaim,
> > > > +
> > > 
> > > If PG_deferred_list implies thp (and replaces the thp dtor), should we
> > > rather name this PG_thp or something along those lines?
> > 
> > We're trying to use 'thp' to mean 'a folio which is pmd mappable',
> > so I'd rather not call it that.
> 
> There is no conclusion on that.

Theree are a lot of counters called THP and TransHuge and other variants
which are exposed to userspace, and the (user) assumption is that this counts
PMD-sized folios.  If you grep around for folio_test_pmd_mappable(),
you'll find them.  If we have folio_test_thp(), people will write:

	if (folio_test_thp(folio))
		__mod_lruvec_state(lruvec, NR_SHMEM_THPS, nr);

instead of using folio_test_pmd_mappable().

> After all, the deferred split queue is just an implementation detail, and it
> happens to live in tailpage 2, no?
> 
> Once we would end up initializing something else in prep_transhuge_page(),
> it would turn out pretty confusing if that is called folio_remove_deferred()
> ...

Perhaps the key difference between normal compound pages and file/anon
compound pages is that the latter are splittable?  So we can name all
of this:

	folio_init_splittable()
	folio_test_splittable()
	folio_fini_splittable()

Maybe that's still too close to an implementation detail, but it's at
least talking about _a_ characteristic of the folio, even if it's not
the _only_ characteristic of the folio.