Re: [PATCH v2 11/15] mm, swap: use the swap table for the swap cache and switch API

[Kairui Song <ryncsn@gmail.com>]

On Sun, Sep 7, 2025 at 8:59 PM Klara Modin <klarasmodin@gmail.com> wrote:
>
> On 2025-09-06 03:13:53 +0800, Kairui Song wrote:
> > From: Kairui Song <kasong@tencent.com>
> >
> > Introduce basic swap table infrastructures, which are now just a
> > fixed-sized flat array inside each swap cluster, with access wrappers.
> >
> > Each cluster contains a swap table of 512 entries. Each table entry is
> > an opaque atomic long. It could be in 3 types: a shadow type (XA_VALUE),
> > a folio type (pointer), or NULL.
> >
> > In this first step, it only supports storing a folio or shadow, and it
> > is a drop-in replacement for the current swap cache. Convert all swap
> > cache users to use the new sets of APIs. Chris Li has been suggesting
> > using a new infrastructure for swap cache for better performance, and
> > that idea combined well with the swap table as the new backing
> > structure. Now the lock contention range is reduced to 2M clusters,
> > which is much smaller than the 64M address_space. And we can also drop
> > the multiple address_space design.
> >
> > All the internal works are done with swap_cache_get_* helpers. Swap
> > cache lookup is still lock-less like before, and the helper's contexts
> > are same with original swap cache helpers. They still require a pin
> > on the swap device to prevent the backing data from being freed.
> >
> > Swap cache updates are now protected by the swap cluster lock
> > instead of the Xarray lock. This is mostly handled internally, but new
> > __swap_cache_* helpers require the caller to lock the cluster. So, a
> > few new cluster access and locking helpers are also introduced.
> >
> > A fully cluster-based unified swap table can be implemented on top
> > of this to take care of all count tracking and synchronization work,
> > with dynamic allocation. It should reduce the memory usage while
> > making the performance even better.
> >
> > Co-developed-by: Chris Li <chrisl@kernel.org>
> > Signed-off-by: Chris Li <chrisl@kernel.org>
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > ---
> >  MAINTAINERS          |   1 +
> >  include/linux/swap.h |   2 -
> >  mm/huge_memory.c     |  13 +-
> >  mm/migrate.c         |  19 ++-
> >  mm/shmem.c           |   8 +-
> >  mm/swap.h            | 157 +++++++++++++++++------
> >  mm/swap_state.c      | 289 +++++++++++++++++++------------------------
> >  mm/swap_table.h      |  97 +++++++++++++++
> >  mm/swapfile.c        | 100 +++++++++++----
> >  mm/vmscan.c          |  20 ++-
> >  10 files changed, 458 insertions(+), 248 deletions(-)
> >  create mode 100644 mm/swap_table.h
> >
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 1c8292c0318d..de402ca91a80 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -16226,6 +16226,7 @@ F:    include/linux/swapops.h
> >  F:   mm/page_io.c
> >  F:   mm/swap.c
> >  F:   mm/swap.h
> > +F:   mm/swap_table.h
> >  F:   mm/swap_state.c
> >  F:   mm/swapfile.c
> >
>
> ...
>
> >  #include <linux/swapops.h> /* for swp_offset */
>
> Now that swp_offset() is used in folio_index(), should this perhaps also be
> included for !CONFIG_SWAP?

Hi, Thanks for looking at this series.

>
> >  #include <linux/blk_types.h> /* for bio_end_io_t */
> >
...

> >       if (unlikely(folio_test_swapcache(folio)))
>
> > -             return swap_cache_index(folio->swap);
> > +             return swp_offset(folio->swap);
>
> This is outside CONFIG_SWAP.

Right, but there are users of folio_index that are outside of
CONFIG_SWAP (mm/migrate.c), and swp_offset is also outside of SWAP so
that's OK.

If we wrap it, the CONFIG_SWAP build will fail. I've test !CONFIG_SWAP
build on this patch and after the whole series, it works fine.

We should drop the usage of folio_index in migrate.c, that's not
really related to this series though.

>
> >       return folio->index;
> >  }
>
> ...
>
> Regards,
> Klara Modin
>