[PATCH v3 13/15] mm, swap: remove contention workaround for swap cache

Kairui Song
2025-09-10T16:09:55Z

From: Kairui Song <kasong@tencent.com>

Swap cluster setup will try to shuffle the clusters on initialization.
It was helpful to avoid contention for the swap cache space. The cluster
size (2M) was much smaller than each swap cache space (64M), so
shuffling the cluster means the allocator will try to allocate swap
slots that are in different swap cache spaces for each CPU, reducing the
chance of two CPUs using the same swap cache space, and hence reducing
the contention.

Now, swap cache is managed by swap clusters, this shuffle is pointless.
Just remove it, and clean up related macros.

This also improves the HDD swap performance as shuffling IO is a bad
idea for HDD, and now the shuffling is gone. Test have shown a ~40%
performance gain for HDD [1]:

Doing sequential swap in of 8G data using 8 processes with usemem,
average of 3 test runs:

Before: 1270.91 KB/s per process
After: 1849.54 KB/s per process

Link: https://lore.kernel.org/linux-mm/CAMgjq7AdauQ8=X0zeih2r21QoV=-WWj1hyBxLWRzq74n-C=-Ng@mail.gmail.com/ [1]
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202504241621.f27743ec-lkp@intel.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
---
mm/swap.h | 4 ----
mm/swapfile.c | 32 ++++++++------------------------
mm/zswap.c | 7 +++++--
3 files changed, 13 insertions(+), 30 deletions(-)

diff --git a/mm/swap.h b/mm/swap.h
index adcd85fa8538..fe5c20922082 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -198,10 +198,6 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug);
void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);

/* linux/mm/swap_state.c */
-/* One swap address space for each 64M swap space */
-#define SWAP_ADDRESS_SPACE_SHIFT 14
-#define SWAP_ADDRESS_SPACE_PAGES (1 << SWAP_ADDRESS_SPACE_SHIFT)
-#define SWAP_ADDRESS_SPACE_MASK (SWAP_ADDRESS_SPACE_PAGES - 1)
extern struct address_space swap_space __ro_after_init;
static inline struct address_space *swap_address_space(swp_entry_t entry)
{
diff --git a/mm/swapfile.c b/mm/swapfile.c
index a81ada4a361d..89659928465e 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3203,21 +3203,14 @@ static int setup_swap_map(struct swap_info_struct *si,
return 0;
}

-#define SWAP_CLUSTER_INFO_COLS \
- DIV_ROUND_UP(L1_CACHE_BYTES, sizeof(struct swap_cluster_info))
-#define SWAP_CLUSTER_SPACE_COLS \
- DIV_ROUND_UP(SWAP_ADDRESS_SPACE_PAGES, SWAPFILE_CLUSTER)
-#define SWAP_CLUSTER_COLS \
- max_t(unsigned int, SWAP_CLUSTER_INFO_COLS, SWAP_CLUSTER_SPACE_COLS)
-
static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
union swap_header *swap_header,
unsigned long maxpages)
{
unsigned long nr_clusters = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
struct swap_cluster_info *cluster_info;
- unsigned long i, j, idx;
int err = -ENOMEM;
+ unsigned long i;

cluster_info = kvcalloc(nr_clusters, sizeof(*cluster_info), GFP_KERNEL);
if (!cluster_info)
@@ -3266,22 +3259,13 @@ static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
INIT_LIST_HEAD(&si->frag_clusters[i]);
}

- /*
- * Reduce false cache line sharing between cluster_info and
- * sharing same address space.
- */
- for (j = 0; j < SWAP_CLUSTER_COLS; j++) {
- for (i = 0; i < DIV_ROUND_UP(nr_clusters, SWAP_CLUSTER_COLS); i++) {
- struct swap_cluster_info *ci;
- idx = i * SWAP_CLUSTER_COLS + j;
- ci = cluster_info + idx;
- if (idx >= nr_clusters)
- continue;
- if (ci->count) {
- ci->flags = CLUSTER_FLAG_NONFULL;
- list_add_tail(&ci->list, &si->nonfull_clusters[0]);
- continue;
- }
+ for (i = 0; i < nr_clusters; i++) {
+ struct swap_cluster_info *ci = &cluster_info[i];
+
+ if (ci->count) {
+ ci->flags = CLUSTER_FLAG_NONFULL;
+ list_add_tail(&ci->list, &si->nonfull_clusters[0]);
+ } else {
ci->flags = CLUSTER_FLAG_FREE;
list_add_tail(&ci->list, &si->free_clusters);
}
diff --git a/mm/zswap.c b/mm/zswap.c
index 3dda4310099e..cba7077fda40 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -225,10 +225,13 @@ static bool zswap_has_pool;
* helpers and fwd declarations
**********************************/

+/* One swap address space for each 64M swap space */
+#define ZSWAP_ADDRESS_SPACE_SHIFT 14
+#define ZSWAP_ADDRESS_SPACE_PAGES (1 << ZSWAP_ADDRESS_SPACE_SHIFT)
static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
{
return &zswap_trees[swp_type(swp)][swp_offset(swp)
- >> SWAP_ADDRESS_SPACE_SHIFT];
+ >> ZSWAP_ADDRESS_SPACE_SHIFT];
}

#define zswap_pool_debug(msg, p) \
@@ -1674,7 +1677,7 @@ int zswap_swapon(int type, unsigned long nr_pages)
struct xarray *trees, *tree;
unsigned int nr, i;

- nr = DIV_ROUND_UP(nr_pages, SWAP_ADDRESS_SPACE_PAGES);
+ nr = DIV_ROUND_UP(nr_pages, ZSWAP_ADDRESS_SPACE_PAGES);
trees = kvcalloc(nr, sizeof(*tree), GFP_KERNEL);
if (!trees) {
pr_err("alloc failed, zswap disabled for swap type %d\n", type);
--
2.51.0