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From:   Nhat Pham <nphamcs@gmail.com>
Date:   Thu, 8 Dec 2022 10:07:49 -0800
Message-ID: <CAKEwX=NwbtFeVyL+J+d8hLeRhoBgfhof+w9_heN++J09XToxCg@mail.gmail.com>
Subject: Re: [PATCH v2 2/4] workingset: refactor LRU refault to expose refault
 recency check
To:     Yu Zhao <yuzhao@google.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, bfoster@redhat.com,
        willy@infradead.org, kernel-team@meta.com
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On Mon, Dec 5, 2022 at 6:26 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Mon, Dec 5, 2022 at 7:22 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Mon, Dec 5, 2022 at 5:29 PM Yu Zhao <yuzhao@google.com> wrote:
> > >
> > > On Mon, Dec 5, 2022 at 6:19 PM Nhat Pham <nphamcs@gmail.com> wrote:
> > > >
> > > > On Mon, Dec 5, 2022 at 3:49 PM Yu Zhao <yuzhao@google.com> wrote:
> > > > >
> > > > > On Mon, Dec 5, 2022 at 10:51 AM Nhat Pham <nphamcs@gmail.com> wrote:
> > > > > >
> > > > > > In preparation for computing recently evicted pages in cachestat,
> > > > > > refactor workingset_refault and lru_gen_refault to expose a helper
> > > > > > function that would test if an evicted page is recently evicted.
> > > > > >
> > > > > > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> > > > > > ---
> > > > > >  include/linux/swap.h |   1 +
> > > > > >  mm/workingset.c      | 143 +++++++++++++++++++++++++++++--------------
> > > > > >  2 files changed, 99 insertions(+), 45 deletions(-)
> > > > > >
> > > > > > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > > > > > index a18cf4b7c724..dae6f6f955eb 100644
> > > > > > --- a/include/linux/swap.h
> > > > > > +++ b/include/linux/swap.h
> > > > > > @@ -361,6 +361,7 @@ static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
> > > > > >  }
> > > > > >
> > > > > >  /* linux/mm/workingset.c */
> > > > > > +bool workingset_test_recent(void *shadow, bool file, bool *workingset);
> > > > > >  void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
> > > > > >  void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg);
> > > > > >  void workingset_refault(struct folio *folio, void *shadow);
> > > > > > diff --git a/mm/workingset.c b/mm/workingset.c
> > > > > > index 79585d55c45d..44b331ce3040 100644
> > > > > > --- a/mm/workingset.c
> > > > > > +++ b/mm/workingset.c
> > > > > > @@ -244,6 +244,30 @@ static void *lru_gen_eviction(struct folio *folio)
> > > > > >         return pack_shadow(mem_cgroup_id(memcg), pgdat, token, refs);
> > > > > >  }
> > > > > >
> > > > > > +/*
> > > > > > + * Test if the folio is recently evicted.
> > > > > > + *
> > > > > > + * As a side effect, also populates the references with
> > > > > > + * values unpacked from the shadow of the evicted folio.
> > > > > > + */
> > > > > > +static bool lru_gen_test_recent(void *shadow, bool file, int *memcgid,
> > > > > > +       struct pglist_data **pgdat, unsigned long *token, bool *workingset)
> > > > > > +{
> > > > > > +       struct mem_cgroup *eviction_memcg;
> > > > > > +       struct lruvec *lruvec;
> > > > > > +       struct lru_gen_struct *lrugen;
> > > > > > +       unsigned long min_seq;
> > > > > > +
> > > > > > +       unpack_shadow(shadow, memcgid, pgdat, token, workingset);
> > > > > > +       eviction_memcg = mem_cgroup_from_id(*memcgid);
> > > > > > +
> > > > > > +       lruvec = mem_cgroup_lruvec(eviction_memcg, *pgdat);
> > > > > > +       lrugen = &lruvec->lrugen;
> > > > > > +
> > > > > > +       min_seq = READ_ONCE(lrugen->min_seq[file]);
> > > > > > +       return !((*token >> LRU_REFS_WIDTH) != (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH)));
> > > > > > +}
> > > > >
> > > > > Nit: not refactoring actually looks cleaner to me -- there are only a
> > > > > few lines of duplicated code and you can get rid of 4 parameters
> > > > > including the unused workingset in the next patch.
> > > >
> > > > (resending this because I forgot to forward to the rest of the
> > > > group!)
> > > >
> > > > Thanks for the comment, Yu!
> > > >
> > > > Personally, I prefer refactoring this piece of logic - I do think that
> > > > it is cleaner than copying the logic into the syscall implementation.
> > >
> > > Let me clarify.
> > >
> > > You can add
> > >   lru_gen_test_recent(void *shadow, bool file)
> > > without refactoring the existing
> > >   lru_gen_refault().
> > >
> > > Set the boilerplate aside, you only repeat one line of code, which is
> > > the last line in the new function.
> > >
> > > (The boilerplate code is repeated in many places, and that's why it's
> > > called boilerplate.)
> >
> > Oh maybe I've misunderstood your original comment. Your
> > suggestion is to repeat the eviction recency check in workingset.c
> > (i.e keep the *_gen_refault unchanged - just copy that logic to
> > *_test_recent)?
>
> Correct.

Thanks for the suggestion, Yu! I'll update if there are complications with
this, but otherwise the change will be in v3 of this patch series.