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From:   Kairui Song <ryncsn@gmail.com>
Date:   Thu, 23 Nov 2023 18:52:06 +0800
Message-ID: <CAMgjq7C8o+Jr=oaoo_q+pdUV4wpGk9DZJHX3y5Zt5R4dPErYwQ@mail.gmail.com>
Subject: Re: [PATCH 08/24] mm/swap: check readahead policy per entry
To:     Chris Li <chrisl@kernel.org>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        "Huang, Ying" <ying.huang@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
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Chris Li <chrisl@kernel.org> =E4=BA=8E2023=E5=B9=B411=E6=9C=8821=E6=97=A5=
=E5=91=A8=E4=BA=8C 15:54=E5=86=99=E9=81=93=EF=BC=9A
>
> On Sun, Nov 19, 2023 at 11:48=E2=80=AFAM Kairui Song <ryncsn@gmail.com> w=
rote:
> >
> > From: Kairui Song <kasong@tencent.com>
> >
> > Currently VMA readahead is globally disabled when any rotate disk is
> > used as swap backend. So multiple swap devices are enabled, if a slower
> > hard disk is set as a low priority fallback, and a high performance SSD
> > is used and high priority swap device, vma readahead is disabled global=
ly.
> > The SSD swap device performance will drop by a lot.
> >
> > Check readahead policy per entry to avoid such problem.
> >
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > ---
> >  mm/swap_state.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index ff6756f2e8e4..fb78f7f18ed7 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -321,9 +321,9 @@ static inline bool swap_use_no_readahead(struct swa=
p_info_struct *si, swp_entry_
> >         return data_race(si->flags & SWP_SYNCHRONOUS_IO) && __swap_coun=
t(entry) =3D=3D 1;
> >  }
> >
> > -static inline bool swap_use_vma_readahead(void)
> > +static inline bool swap_use_vma_readahead(struct swap_info_struct *si)
> >  {
> > -       return READ_ONCE(enable_vma_readahead) && !atomic_read(&nr_rota=
te_swap);
> > +       return data_race(si->flags & SWP_SOLIDSTATE) && READ_ONCE(enabl=
e_vma_readahead);
>
> A very minor point:
> I notice you change the order enable_vma_readahead to the last.
> Normally if enable_vma_reachahead =3D=3D 0, there is no need to check the=
 si->flags.
> The si->flags check is more expensive than simple memory load.
> You might want to check enable_vma_readahead first then you can short
> cut the more expensive part.

Thanks, I'll improve this part.