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 1:53 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 11.07.24 21:49, Yu Zhao wrote:
> > 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'.
>
> Great, thanks.
>
> >
> >> 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.
>
> Let me rephrase:
>
> Checking for lazyfree is
>
> "folio_test_anon(folio) && !folio_test_swapbacked(folio)"
>
> Testing for dirtied lazyfree is
>
> "folio_test_anon(folio) && !folio_test_swapbacked(folio) &&
>   folio_test)dirty(folio)"
>
> So I'm wondering about the missing folio_test_swapbacked() test.

It's not missing: type == LRU_GEN_FILE means folio_is_file_lru(),
which in turn means !folio_test_swapbacked().