mm/swap: reduce swap cache search space

Currently we use one swap_address_space for every 64M chunk to reduce lock
contention, this is like having a set of smaller swap files inside one
swap device.  But when doing swap cache look up or insert, we are still
using the offset of the whole large swap device.  This is OK for
correctness, as the offset (key) is unique.

But Xarray is specially optimized for small indexes, it creates the radix
tree levels lazily to be just enough to fit the largest key stored in one
Xarray.  So we are wasting tree nodes unnecessarily.

For 64M chunk it should only take at most 3 levels to contain everything. 
But if we are using the offset from the whole swap device, the offset
(key) value will be way beyond 64M, and so will the tree level.

Optimize this by using a new helper swap_cache_index to get a swap entry's
unique offset in its own 64M swap_address_space.

I see a ~1% performance gain in benchmark and actual workload with high
memory pressure.

Test with `time memhog 128G` inside a 8G memcg using 128G swap (ramdisk
with SWP_SYNCHRONOUS_IO dropped, tested 3 times, results are stable.  The
test result is similar but the improvement is smaller if
SWP_SYNCHRONOUS_IO is enabled, as swap out path can never skip swap
cache):

Before:
6.07user 250.74system 4:17.26elapsed 99%CPU (0avgtext+0avgdata 8373376maxresident)k
0inputs+0outputs (55major+33555018minor)pagefaults 0swaps

After (1.8% faster):
6.08user 246.09system 4:12.58elapsed 99%CPU (0avgtext+0avgdata 8373248maxresident)k
0inputs+0outputs (54major+33555027minor)pagefaults 0swaps

Similar result with MySQL and sysbench using swap:
Before:
94055.61 qps

After (0.8% faster):
94834.91 qps

Radix tree slab usage is also very slightly lower.

Link: https://lkml.kernel.org/r/20240521175854.96038-12-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chao Yu <chao@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Xiubo Li <xiubli@redhat.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 9 insertions, 8 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 6498491e3ad8..a5dae40523ab 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -72,7 +72,7 @@ void show_swap_cache_info(void)
 void *get_shadow_from_swap_cache(swp_entry_t entry)
 {
 	struct address_space *address_space = swap_address_space(entry);
-	pgoff_t idx = swp_offset(entry);
+	pgoff_t idx = swap_cache_index(entry);
 	void *shadow;
 
 	shadow = xa_load(&address_space->i_pages, idx);
@@ -89,7 +89,7 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
 			gfp_t gfp, void **shadowp)
 {
 	struct address_space *address_space = swap_address_space(entry);
-	pgoff_t idx = swp_offset(entry);
+	pgoff_t idx = swap_cache_index(entry);
 	XA_STATE_ORDER(xas, &address_space->i_pages, idx, folio_order(folio));
 	unsigned long i, nr = folio_nr_pages(folio);
 	void *old;
@@ -144,7 +144,7 @@ void __delete_from_swap_cache(struct folio *folio,
 	struct address_space *address_space = swap_address_space(entry);
 	int i;
 	long nr = folio_nr_pages(folio);
-	pgoff_t idx = swp_offset(entry);
+	pgoff_t idx = swap_cache_index(entry);
 	XA_STATE(xas, &address_space->i_pages, idx);
 
 	xas_set_update(&xas, workingset_update_node);
@@ -253,13 +253,14 @@ void clear_shadow_from_swap_cache(int type, unsigned long begin,
 
 	for (;;) {
 		swp_entry_t entry = swp_entry(type, curr);
+		unsigned long index = curr & SWAP_ADDRESS_SPACE_MASK;
 		struct address_space *address_space = swap_address_space(entry);
-		XA_STATE(xas, &address_space->i_pages, curr);
+		XA_STATE(xas, &address_space->i_pages, index);
 
 		xas_set_update(&xas, workingset_update_node);
 
 		xa_lock_irq(&address_space->i_pages);
-		xas_for_each(&xas, old, end) {
+		xas_for_each(&xas, old, min(index + (end - curr), SWAP_ADDRESS_SPACE_PAGES)) {
 			if (!xa_is_value(old))
 				continue;
 			xas_store(&xas, NULL);
@@ -350,7 +351,7 @@ struct folio *swap_cache_get_folio(swp_entry_t entry,
 {
 	struct folio *folio;
 
-	folio = filemap_get_folio(swap_address_space(entry), swp_offset(entry));
+	folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry));
 	if (!IS_ERR(folio)) {
 		bool vma_ra = swap_use_vma_readahead();
 		bool readahead;
@@ -420,7 +421,7 @@ struct folio *filemap_get_incore_folio(struct address_space *mapping,
 	si = get_swap_device(swp);
 	if (!si)
 		return ERR_PTR(-ENOENT);
-	index = swp_offset(swp);
+	index = swap_cache_index(swp);
 	folio = filemap_get_folio(swap_address_space(swp), index);
 	put_swap_device(si);
 	return folio;
@@ -447,7 +448,7 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * that would confuse statistics.
 		 */
 		folio = filemap_get_folio(swap_address_space(entry),
-						swp_offset(entry));
+					  swap_cache_index(entry));
 		if (!IS_ERR(folio))
 			goto got_folio;