Re: [PATCH v9 05/14] mm: multi-gen LRU: groundwork

[Barry Song <21cnbao@gmail.com>]

On Mon, Mar 21, 2022 at 10:04 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Wed, Mar 16, 2022 at 5:25 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> ...
> > > +static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming)
> > > +{
> > > +       int gen;
> > > +       unsigned long old_flags, new_flags;
> > > +       int type = folio_is_file_lru(folio);
> > > +       int zone = folio_zonenum(folio);
> > > +       struct lru_gen_struct *lrugen = &lruvec->lrugen;
> > > +
> > > +       if (folio_test_unevictable(folio))
> > > +               return false;
> > > +       /*
> > > +        * There are three common cases for this page:
> > > +        * 1. If it's hot, e.g., freshly faulted in or previously hot and
> > > +        *    migrated, add it to the youngest generation.
> >
> > usually, one page is not active when it is faulted in. till its second
> > access is detected, it can be active.
>
> The active/inactive LRU *assumes* this; MGLRU *assumes* the opposite,
> and there is no "active" in MGLRU -- we call it hot to avoid confusion
> :)

yep.

>
> > > +        * 2. If it's cold but can't be evicted immediately, i.e., an anon page
> > > +        *    not in swapcache or a dirty page pending writeback, add it to the
> > > +        *    second oldest generation.
> > > +        * 3. Everything else (clean, cold) is added to the oldest generation.
> > > +        */
> ...
> > > +#define LRU_GEN_MASK           ((BIT(LRU_GEN_WIDTH) - 1) << LRU_GEN_PGOFF)
> > > +#define LRU_REFS_MASK          ((BIT(LRU_REFS_WIDTH) - 1) << LRU_REFS_PGOFF)
> >
> > The commit log said nothing about REFS flags and tiers.
> > but the code is here. either the commit log lacks something
> > or the code should belong to the next patch?
>
> It did:
>   A few macros, i.e., LRU_REFS_*, used later are added in this patch
> to make the patchset less diffy.

sorry for missing that.

>
> > > @@ -462,6 +462,11 @@ void folio_add_lru(struct folio *folio)
> > >         VM_BUG_ON_FOLIO(folio_test_active(folio) && folio_test_unevictable(folio), folio);
> > >         VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
> > >
> > > +       /* see the comment in lru_gen_add_folio() */
> > > +       if (lru_gen_enabled() && !folio_test_unevictable(folio) &&
> > > +           lru_gen_in_fault() && !(current->flags & PF_MEMALLOC))
> > > +               folio_set_active(folio);
> >
> > So here is our magic to make folio active as long as it is
> > faulted in? i really don't think the below comment is good,
> > could we say our purpose directly and explicitly?
> >
> >  /* see the comment in lru_gen_add_folio() */
>
> I generally keep comments in a few major locations and reference them
> from many other minior locations so that it's easier to manage in the
> long run. It is a hassle for reviews but once in the tree you can jump
> to lru_gen_add_folio() with ctags/cscope or find all places that
> reference it by grepping. Assuming we state the purpose, which is to
> make lru_gen_add_folio() add the page to the youngest generation, you
> still want to go to lru_gen_add_folio() to check if this is really the
> case. So why bother :)

well understood though my pain was that I needed to email you to get
confirmed this is really the case.

Thanks
Barry