Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

From: Yu Zhao <yuzhao@google.com>
To: Barry Song <21cnbao@gmail.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
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Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation
Date: Thu, 24 Mar 2022 00:24:40 -0600	[thread overview]
Message-ID: <CAOUHufazsjhp3GzV7QFOqEeG98zJyRWd+HocyYR1FP3nk-Ho0Q@mail.gmail.com> (raw)
In-Reply-To: <CAGsJ_4zeNtAsq7E9SKVwuKaXnKj80BP2NPHdjtd1i=098M6eDw@mail.gmail.com>

On Wed, Mar 23, 2022 at 1:47 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Sat, Mar 19, 2022 at 4:11 PM Yu Zhao <yuzhao@google.com> wrote:
> >
> > On Fri, Mar 18, 2022 at 9:01 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > > +static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio, bool reclaiming)
> > > > +{
> > > > +       unsigned long old_flags, new_flags;
> > > > +       int type = folio_is_file_lru(folio);
> > > > +       struct lru_gen_struct *lrugen = &lruvec->lrugen;
> > > > +       int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]);
> > > > +
> > > > +       do {
> > > > +               new_flags = old_flags = READ_ONCE(folio->flags);
> > > > +               VM_BUG_ON_FOLIO(!(new_flags & LRU_GEN_MASK), folio);
> > > > +
> > > > +               new_gen = ((new_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> > > > +               new_gen = (old_gen + 1) % MAX_NR_GENS;
> > >
> > > new_gen is assigned twice, i assume you mean
> > >                old_gen = ((new_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> > >                new_gen = (old_gen + 1) % MAX_NR_GENS;
> > >
> > > or do you always mean new_gen =  lru_gen_from_seq(min_seq) + 1?
> >
> > Thanks a lot for your attention to details!
> >
> > The first line should be in the next patch but I overlooked during the
> > last refactoring:
>
> Thanks for the clarification. So an unmapped file-backed page which is
> accessed only by system call will always be in either min_seq or
> min_seq + 1? it has no chance to be in max_seq like a faulted-in
> mapped file page?

That's right. The rationale is documented here under the `Assumptions`
section [1]. This is also related to Aneesh's question about why MGLRU
doesn't need additional heuristics for VM_EXEC pages [2]. Unmapped
file pages weaken the protection of executable pages under heavy
buffered IO workloads like Java NIO.

[1] https://lore.kernel.org/linux-mm/20220309021230.721028-15-yuzhao@google.com/
[2] https://lore.kernel.org/linux-mm/CAOUHufYfpiGdLSdffvzDqaD5oYFG99oDJ2xgQd2Ph77OFR5NAA@mail.gmail.com/