Re: [PATCH v2 05/15] mm, swap: always lock and check the swap cache folio before use

[Kairui Song <ryncsn@gmail.com>]

On Sat, Sep 6, 2025 at 11:51 AM Chris Li <chrisl@kernel.org> wrote:
>
> Looks correct to me.
>
> Acked-by: Chris Li <chrisl@kernel.org>

Thanks.

>
> With some nitpick follows,
>
> On Fri, Sep 5, 2025 at 12:14 PM Kairui Song <ryncsn@gmail.com> wrote:
> >
> > From: Kairui Song <kasong@tencent.com>
> >
> > Swap cache lookup only increases the reference count of the returned
> > folio. That's not enough to ensure a folio is stable in the swap
> > cache, so the folio could be removed from the swap cache at any
> > time. The caller should always lock and check the folio before using it.
> >
> > We have just documented this in kerneldoc, now introduce a helper for swap
> > cache folio verification with proper sanity checks.
> >
> > Also, sanitize a few current users to use this convention and the new
> > helper for easier debugging. They were not having observable problems
> > yet, only trivial issues like wasted CPU cycles on swapoff or
> > reclaiming. They would fail in some other way, but it is still better to
> > always follow this convention to make things robust and make later
> > commits easier to do.
> >
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > ---
> >  mm/memory.c     |  3 +--
> >  mm/swap.h       | 24 ++++++++++++++++++++++++
> >  mm/swap_state.c |  7 +++++--
> >  mm/swapfile.c   | 10 +++++++---
> >  4 files changed, 37 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 94a5928e8ace..5808c4ef21b3 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4748,8 +4748,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >                  * swapcache, we need to check that the page's swap has not
> >                  * changed.
> >                  */
> > -               if (unlikely(!folio_test_swapcache(folio) ||
> > -                            page_swap_entry(page).val != entry.val))
> > +               if (unlikely(!folio_matches_swap_entry(folio, entry)))
> >                         goto out_page;
> >
> >                 if (unlikely(PageHWPoison(page))) {
> > diff --git a/mm/swap.h b/mm/swap.h
> > index efb6d7ff9f30..a69e18b12b45 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -52,6 +52,25 @@ static inline pgoff_t swap_cache_index(swp_entry_t entry)
> >         return swp_offset(entry) & SWAP_ADDRESS_SPACE_MASK;
> >  }
> >
> > +/**
> > + * folio_matches_swap_entry - Check if a folio matches a given swap entry.
> > + * @folio: The folio.
> > + * @entry: The swap entry to check against.
> > + *
> > + * Context: The caller should have the folio locked to ensure it's stable
> > + * and nothing will move it in or out of the swap cache.
> > + * Return: true or false.
> > + */
> > +static inline bool folio_matches_swap_entry(const struct folio *folio,
> > +                                           swp_entry_t entry)
> > +{
> > +       VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
> > +       if (!folio_test_swapcache(folio))
> > +               return false;
> > +       VM_WARN_ON_ONCE_FOLIO(!IS_ALIGNED(folio->swap.val, folio_nr_pages(folio)), folio);
>
> You should cache the folio->swap.val into a local register value.
> Because WARN_ON_ONCE I think the compiler has no choice but to load it
> twice? Haven't verified it myself.
>
> There is no downside in compiler point of view using more local
> variables, the compiler generates an internal version of the local
> variable equivalent anyway.
>
> > +       return folio->swap.val == round_down(entry.val, folio_nr_pages(folio));
>
> Same for folio_nr_pages(folio), you should cache it. The function will
> look less busy.

That's a very good idea, that should also reduce line length so it is
easier to read.