Re: [PATCH v2 03/15] mm, swap: fix swap cahe index error when retrying reclaim

[Chris Li <chrisl@kernel.org>]

On Fri, Sep 5, 2025 at 11:29 PM Kairui Song <ryncsn@gmail.com> wrote:
>
> On Sat, Sep 6, 2025 at 11:19 AM Chris Li <chrisl@kernel.org> wrote:
> >
> > Hi Kairui,
> >
> > The patch looks obviously correct to me with some very minor nitpicks following.
> >
> > Acked-by: Chris Li <chrisl@kernel.org>
> >
> > On Fri, Sep 5, 2025 at 12:14 PM Kairui Song <ryncsn@gmail.com> wrote:
> > >
> > > From: Kairui Song <kasong@tencent.com>
> > >
> > > The allocator will reclaim cached slots while scanning. Currently, it
> > > will try again if the reclaim found a folio that is already removed from
> > > the swap cache due to a race. But the following lookup will be using the
> > > wrong index. It won't cause any OOB issue since the swap cache index is
> > > truncated upon lookup, but it may lead to reclaiming of an irrelevant
> > > folio.
> > >
> > > This should not cause a measurable issue, but we should fix it.
> > >
> > > Fixes: fae8595505313 ("mm, swap: avoid reclaiming irrelevant swap cache")
> > > Signed-off-by: Kairui Song <kasong@tencent.com>
> > > ---
> > >  mm/swapfile.c | 6 +++---
> > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > > index 4b8ab2cb49ca..4c63fc62f4cb 100644
> > > --- a/mm/swapfile.c
> > > +++ b/mm/swapfile.c
> > > @@ -240,13 +240,13 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> > >          * Offset could point to the middle of a large folio, or folio
> > >          * may no longer point to the expected offset before it's locked.
> > >          */
> > > -       entry = folio->swap;
> > Nitpick:
> > This and the following reuse the folio->swap dereference and
> > swp_offset() many times.
> > You can use some local variables to cache the value into a register
> > and less function calls. I haven't looked into if the compiler will do
> > the same expression elimination on this, a good compiler should. The
> > following looks less busy and doesn't need the compiler to optimize it
> > for you.
> >
> >            fe = folio->swap;
> >            eoffset = swp_offset(fe);
> >            if (offset < eoffset ) || offset >= eoffset + nr_pages) {
> > ...
> >            }
> >            offset = eoffset;
> >
> > This might generate better code due to less function code. If the
> > compiler does the perfect jobs the original code can generate the same
> > optimized code as well.
>
> Right, this part of the code will be gone soon so I think maybe better
> to keep the change minimal, and it's not a hot path.

Ack. It is nitpick anyway. Most likely doesn't make a difference to
modern compilers anyway.

Chris