linux-kernel - Re: [PATCH 2/4] mm/shmem, swap: avoid redundant Xarray lookup during swapin

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Message-ID: <CAMgjq7BmJ1fqWsDgkbiCt++_uA1D0kxfDOdKVjtHCS9Y4jfKWg@mail.gmail.com>
Date: Wed, 18 Jun 2025 15:22:44 +0800
From: Kairui Song <ryncsn@...il.com>
To: Dev Jain <dev.jain@....com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Hugh Dickins <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Matthew Wilcox <willy@...radead.org>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Chris Li <chrisl@...nel.org>, Nhat Pham <nphamcs@...il.com>, Baoquan He <bhe@...hat.com>, 
	Barry Song <baohua@...nel.org>, linux-kernel@...r.kernel.org, 
	Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH 2/4] mm/shmem, swap: avoid redundant Xarray lookup during swapin

On Wed, Jun 18, 2025 at 3:17 PM Dev Jain <dev.jain@....com> wrote:
>
>
> On 18/06/25 12:05 am, Kairui Song wrote:
> > From: Kairui Song <kasong@...cent.com>
> >
> > Currently shmem calls xa_get_order to get the swap radix entry order,
> > requiring a full tree walk. This can be easily combined with the swap
> > entry value checking (shmem_confirm_swap) to avoid the duplicated
> > lookup, which should improve the performance.
>
> Nice spot!
>
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > ---
> >   mm/shmem.c | 33 ++++++++++++++++++++++++---------
> >   1 file changed, 24 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 4e7ef343a29b..0ad49e57f736 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -505,15 +505,27 @@ static int shmem_replace_entry(struct address_space *mapping,
> >
> >   /*
> >    * Sometimes, before we decide whether to proceed or to fail, we must check
> > - * that an entry was not already brought back from swap by a racing thread.
> > + * that an entry was not already brought back or split by a racing thread.
> >    *
> >    * Checking folio is not enough: by the time a swapcache folio is locked, it
> >    * might be reused, and again be swapcache, using the same swap as before.
> > + * Returns the swap entry's order if it still presents, else returns -1.
> >    */
> > -static bool shmem_confirm_swap(struct address_space *mapping,
> > -                            pgoff_t index, swp_entry_t swap)
> > +static int shmem_swap_check_entry(struct address_space *mapping, pgoff_t index,
> > +                               swp_entry_t swap)
>
> I think the function name shmem_confirm_swap is already good enough? Anyhow the
> changed name should at least be shmem_check_entry_is_swap.
>

Good, I can keep the function name unchanged or follow your
suggestion, I thought a `confirm` function returning non-binary return
value may look strange. I'm terrible at naming things :P