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From:   Yu Zhao <yuzhao@google.com>
Date:   Mon, 21 Mar 2022 03:04:22 -0600
Message-ID: <CAOUHufYPT3-_5sAX9r0hyZz4As79FKnVP=gOT=weFdYje0bR5Q@mail.gmail.com>
Subject: Re: [PATCH v9 05/14] mm: multi-gen LRU: groundwork
To:     Barry Song <21cnbao@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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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
:)

> > +        * 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.

> > @@ -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 :)