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

[Yu Zhao <yuzhao@google.com>]

On Thu, Jul 11, 2024 at 1:20 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 11.07.24 21:18, David Hildenbrand wrote:
> > 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;

This is an optimization to avoid an unnecessary trip to
shrink_folio_list(), so it's safe to delete the entire 'if' block, and
that would be preferable than leaving a dangling 'if'.

> Note that something is unclear to me: are we maybe running into that
> code also if folio_set_swapbacked() is already set and we are not in the
> lazyfree path (in contrast to what is documented)?

Not sure what you mean: either rmap sees pte_dirty() and does
folio_mark_dirty() and then folio_set_swapbacked(); or MGLRU does the
same sequence, with the first two steps in walk_pte_range() and the
last one here.