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Message-ID: <eu3s6hqfcbkymjqk2rrngzx7qxjzpivfty2dr4lrduxohxuuuv@g63qihvder5d>
Date: Mon, 8 Sep 2025 17:00:58 +0200
From: Klara Modin <klarasmodin@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>, Chris Li <chrisl@...nel.org>,
Barry Song <baohua@...nel.org>, Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>,
Kemeng Shi <shikemeng@...weicloud.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
Ying Huang <ying.huang@...ux.alibaba.com>, Johannes Weiner <hannes@...xchg.org>,
David Hildenbrand <david@...hat.com>, Yosry Ahmed <yosryahmed@...gle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 11/15] mm, swap: use the swap table for the swap cache
and switch API
On 2025-09-08 22:34:04 +0800, Kairui Song wrote:
> On Sun, Sep 7, 2025 at 8:59 PM Klara Modin <klarasmodin@...il.com> wrote:
> >
> > On 2025-09-06 03:13:53 +0800, Kairui Song wrote:
> > > From: Kairui Song <kasong@...cent.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@...nel.org>
> > > Signed-off-by: Chris Li <chrisl@...nel.org>
> > > Signed-off-by: Kairui Song <kasong@...cent.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.
Interesting that it works for you. I have a config with !CONFIG_SWAP which
fails with:
In file included from mm/shmem.c:44:
mm/swap.h: In function ‘folio_index’:
mm/swap.h:461:24: error: implicit declaration of function ‘swp_offset’; did you mean ‘pmd_offset’? [-Wimplicit-function-declaration]
461 | return swp_offset(folio->swap);
| ^~~~~~~~~~
| pmd_offset
(though it's possible I have misapplied the series somehow).
If I just move the linux/swapops.h include outside the CONFIG_SWAP ifdef:
diff --git a/mm/swap.h b/mm/swap.h
index caff4fe30fc5..12dd7d6478ff 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -3,6 +3,7 @@
#define _MM_SWAP_H
#include <linux/atomic.h> /* for atomic_long_t */
+#include <linux/swapops.h> /* for swp_offset */
struct mempolicy;
struct swap_iocb;
@@ -54,7 +55,6 @@ enum swap_cluster_flags {
};
#ifdef CONFIG_SWAP
-#include <linux/swapops.h> /* for swp_offset */
#include <linux/blk_types.h> /* for bio_end_io_t */
static inline unsigned int swp_cluster_offset(swp_entry_t entry)
it fixes that issue for me, and my other CONFIG_SWAP builds do not seem
to be impacted. I attached the config in case it's useful.
>
> >
> > > return folio->index;
> > > }
> >
> > ...
> >
> > Regards,
> > Klara Modin
> >
Download attachment "config.gz" of type "application/gzip" (10332 bytes)
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