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

[Kairui Song <ryncsn@gmail.com>]

On Mon, Sep 8, 2025 at 10:39 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 08.09.25 16:20, Kairui Song wrote:
> > On Mon, Sep 8, 2025 at 8:35 PM David Hildenbrand <david@redhat.com> 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.
>
> Right, but it's probably worth it.
>
> >
> > 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.
>
> Well, the current API as you introduce it here is confusing, as it's not
> clear who is supposed to initialize what.
>
> So better to it cleanly right from the start.
>
> >
> >> 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`.
>
> Well, no other swap_cache_* functions consumes an address space, right?

Right. I can drop it in this patch.

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