linux-kernel - Re: [PATCH -V2 1/5] swap: Remove get/put_swap_device() in __swap

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Date:   Mon, 22 May 2023 18:51:10 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Minchan Kim <minchan@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH -V2 1/5] swap: Remove get/put_swap_device() in __swap_count()

On Mon, May 22, 2023 at 6:48 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Yosry Ahmed <yosryahmed@...gle.com> writes:
>
> > On Mon, May 22, 2023 at 12:09 AM Huang Ying <ying.huang@...el.com> wrote:
> >>
> >> __swap_count() is called in do_swap_page() only, which encloses the
> >> call site with get/put_swap_device() already.
> >>
> >> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> >> Cc: David Hildenbrand <david@...hat.com>
> >> Cc: Hugh Dickins <hughd@...gle.com>
> >> Cc: Johannes Weiner <hannes@...xchg.org>
> >> Cc: Matthew Wilcox <willy@...radead.org>
> >> Cc: Michal Hocko <mhocko@...e.com>
> >> Cc: Minchan Kim <minchan@...nel.org>
> >> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> >> Cc: Yang Shi <shy828301@...il.com>
> >> Cc: Yu Zhao <yuzhao@...gle.com>
> >> ---
> >>  mm/swapfile.c | 10 ++--------
> >>  1 file changed, 2 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >> index 274bbf797480..8419cba9c192 100644
> >> --- a/mm/swapfile.c
> >> +++ b/mm/swapfile.c
> >> @@ -1432,16 +1432,10 @@ void swapcache_free_entries(swp_entry_t *entries, int n)
> >>
> >
> > nit: I would add a comment here that the caller needs get/put_swap_device().
>
> It's default behavior to call get/put_swap_device() in the caller for
> all almost all swap functions.  I would rather comment the swap
> functions needn't to do that, as the comments for
> read_swap_cache_async() in [2/5].

Fair enough. The comment that you added above get_swap_device() states
that all swap-related functions should have some sort of protection
against swapoff, so I guess that's sufficient.

My concern is that sometimes people don't know where to look, and
having a comment above the function might be helpful. I do agree
though that having a comment above ~all swap-related functions
pointing to the comment above get_swap_device() is too much.

>
> > Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>
>
> Thanks!
>
> >>  int __swap_count(swp_entry_t entry)
> >>  {
> >> -       struct swap_info_struct *si;
> >> +       struct swap_info_struct *si = swp_swap_info(entry);
> >>         pgoff_t offset = swp_offset(entry);
> >> -       int count = 0;
> >>
> >> -       si = get_swap_device(entry);
> >> -       if (si) {
> >> -               count = swap_count(si->swap_map[offset]);
> >> -               put_swap_device(si);
> >> -       }
> >> -       return count;
> >> +       return swap_count(si->swap_map[offset]);
> >>  }
> >>
> >>  /*
>
> Best Regards,
> Huang, Ying