Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

[David Hildenbrand]

On 11.07.24 20:56, David Hildenbrand wrote:
> On 11.07.24 20:54, Jason A. Donenfeld wrote:
> > On Thu, Jul 11, 2024 at 08:24:07PM +0200, David Hildenbrand wrote:
> > > > And PG_large_rmappable seems to only be used for hugetlb branches.
> > >
> > > It should be set for THP/large folios.
> >
> > And it's tested too, apparently.
> >
> > Okay, well, how disappointing is this below? Because I'm running out of
> > tricks for flag reuse.
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index b9e914e1face..c1ea49a7f198 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -110,6 +110,7 @@ enum pageflags {
> >    	PG_workingset,
> >    	PG_error,
> >    	PG_owner_priv_1,	/* Owner use. If pagecache, fs may use*/
> > +	PG_owner_priv_2,
>
> Oh no, no new page flags please :)
>
> Maybe just follow what Linux suggested: pass vma to pte_dirty() and
> always return false for these special VMAs.

... or look into removing that one case that gives us headake.

No idea what would happen if we do the following:

CCing Yu Zhao.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0761f91b407f..d1dfbd4fd38d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4280,14 +4280,9 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
                return true;
        }
 
-       /* dirty lazyfree */
-       if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio)) {
-               success = lru_gen_del_folio(lruvec, folio, true);
-               VM_WARN_ON_ONCE_FOLIO(!success, folio);
-               folio_set_swapbacked(folio);
-               lruvec_add_folio_tail(lruvec, folio);
-               return true;
-       }
+       /* lazyfree: we may not be allowed to set swapbacked: MAP_DROPPABLE */
+       if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio))
+               return false;
 


--
Cheers,

David / dhildenb