Re: [PATCH 08/24] mm/swap: check readahead policy per entry

[Kairui Song <ryncsn@gmail.com>]

Chris Li <chrisl@kernel.org> 于2023年11月21日周二 15:54写道：
>
> On Sun, Nov 19, 2023 at 11:48 AM Kairui Song <ryncsn@gmail.com> wrote:
> >
> > 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 globally.
> > 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 swap_info_struct *si, swp_entry_
> >         return data_race(si->flags & SWP_SYNCHRONOUS_IO) && __swap_count(entry) == 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_rotate_swap);
> > +       return data_race(si->flags & SWP_SOLIDSTATE) && READ_ONCE(enable_vma_readahead);
>
> A very minor point:
> I notice you change the order enable_vma_readahead to the last.
> Normally if enable_vma_reachahead == 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.