Re: [PATCH v2 10/15] mm, swap: wrap swap cache replacement with a helper

[Kairui Song]

On Mon, Sep 8, 2025 at 8:35 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
>
> >
> > +/**
> > + * __swap_cache_replace_folio - Replace a folio in the swap cache.
> > + * @mapping: Swap mapping address space.
> > + * @entry: The first swap entry that the new folio corresponds to.
> > + * @old: The old folio to be replaced.
> > + * @new: The new folio.
> > + *
> > + * Replace a existing folio in the swap cache with a new folio.
> > + *
> > + * Context: Caller must ensure both folios are locked, and lock the
> > + * swap address_space that holds the entries to be replaced.
> > + */
> > +void __swap_cache_replace_folio(struct address_space *mapping,
> > +                             swp_entry_t entry,
> > +                             struct folio *old, struct folio *new)
>
> Can't we just use "new->swap.val" directly and avoid passing in the
> entry, documenting that new->swap.val must be setup properly in advance?

Thanks for the suggestion.

I was thinking about the opposite. I think maybe it's better that the
caller never sets the new folio's entry value, so folio->swap is always
modified in mm/swap_state.c, and let __swap_cache_replace_folio set
new->swap, to make it easier to track the folio->swap
usage.

This can be done easily for migration and shmem parts, the huge split
code will need a bit more cleanup.

It's a trivial change I think. But letting __swap_cache_replace_folio
setup new's swap and flags may deduplicate some code. So I thought
maybe this can be better cleaned up later. So for now I just add a
debug check here that `entry == new->swap`.

And the debug check does imply that we can just drop the entry params
in this patch, there will be no feature change.

> Similarly, can't we obtain "mapping" from new?

This is doable. But this patch is only an intermediate patch, next
commit will let the pass in ci instead. Of course the `ci` can be
retrieved from `entry` directly too, but it's the caller's
responsibility to lock the `ci`, so passing in a locked ci explicitly
might be more intuitive? Also might save a tiny bit of CPU time from
recalculating and load the `ci`.


>
> --
> Cheers
>
> David / dhildenb
>
>