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

[David Hildenbrand]

On 11.07.24 21:58, Yu Zhao wrote:
> On Thu, Jul 11, 2024 at 1:53 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 11.07.24 21:49, Yu Zhao wrote:
> > > On Thu, Jul 11, 2024 at 1:20 PM David Hildenbrand <david@xxxxxxxxxx> 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().

Ahh, got it, thanks!

--
Cheers,

David / dhildenb