[PATCH] mm: attempt to batch free swap entries for zap_pte_range()

[Barry Song <21cnbao@gmail.com>]

From: Barry Song <v-songbaohua@oppo.com>

Zhiguo reported that swap release could be a serious bottleneck
during process exits[1]. With mTHP, we have the opportunity to
batch free swaps.
Thanks to the work of Chris and Kairui[2], I was able to achieve
this optimization with minimal code changes by building on their
efforts.
If swap_count is 1, which is likely true as most anon memory are
private, we can free all contiguous swap slots all together.

Ran the below test program for measuring the bandwidth of munmap
using zRAM and 64KiB mTHP:

 #include <sys/mman.h>
 #include <sys/time.h>
 #include <stdlib.h>

 unsigned long long tv_to_ms(struct timeval tv)
 {
        return tv.tv_sec * 1000 + tv.tv_usec / 1000;
 }

 main()
 {
        struct timeval tv_b, tv_e;
        int i;
 #define SIZE 1024*1024*1024
        void *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
                                MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        if (!p) {
                perror("fail to get memory");
                exit(-1);
        }

        madvise(p, SIZE, MADV_HUGEPAGE);
        memset(p, 0x11, SIZE); /* write to get mem */

        madvise(p, SIZE, MADV_PAGEOUT);

        gettimeofday(&tv_b, NULL);
        munmap(p, SIZE);
        gettimeofday(&tv_e, NULL);

        printf("munmap in bandwidth: %ld bytes/ms\n",
                        SIZE/(tv_to_ms(tv_e) - tv_to_ms(tv_b)));
 }

The result is as below (munmap bandwidth):
                mm-unstable  mm-unstable-with-patch
   round1       21053761      63161283
   round2       21053761      63161283
   round3       21053761      63161283
   round4       20648881      67108864
   round5       20648881      67108864

munmap bandwidth becomes 3X faster.

[1] https://lore.kernel.org/linux-mm/20240731133318.527-1-justinjiang@vivo.com/
[2] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-0-cb9c148b9297@kernel.org/

Cc: Kairui Song <kasong@tencent.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 mm/swapfile.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index ea023fc25d08..ed872a186e81 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -156,6 +156,25 @@ static bool swap_is_has_cache(struct swap_info_struct *si,
 	return true;
 }
 
+static bool swap_is_last_map(struct swap_info_struct *si,
+			      unsigned long offset, int nr_pages,
+			      bool *has_cache)
+{
+	unsigned char *map = si->swap_map + offset;
+	unsigned char *map_end = map + nr_pages;
+	bool cached = false;
+
+	do {
+		if ((*map & ~SWAP_HAS_CACHE) != 1)
+			return false;
+		if (*map & SWAP_HAS_CACHE)
+			cached = true;
+	} while (++map < map_end);
+
+	*has_cache = cached;
+	return true;
+}
+
 /*
  * returns number of pages in the folio that backs the swap entry. If positive,
  * the folio was reclaimed. If negative, the folio was not reclaimed. If 0, no
@@ -1469,6 +1488,39 @@ static unsigned char __swap_entry_free(struct swap_info_struct *p,
 	return usage;
 }
 
+static bool try_batch_swap_entries_free(struct swap_info_struct *p,
+		swp_entry_t entry, int nr, bool *any_only_cache)
+{
+	unsigned long offset = swp_offset(entry);
+	struct swap_cluster_info *ci;
+	bool has_cache = false;
+	bool can_batch;
+	int i;
+
+	/* cross into another cluster */
+	if (nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER)
+		return false;
+	ci = lock_cluster_or_swap_info(p, offset);
+	can_batch = swap_is_last_map(p, offset, nr, &has_cache);
+	if (can_batch) {
+		for (i = 0; i < nr; i++)
+			WRITE_ONCE(p->swap_map[offset + i], SWAP_HAS_CACHE);
+	}
+	unlock_cluster_or_swap_info(p, ci);
+
+	/* all swap_maps have count==1 and have no swapcache */
+	if (!can_batch)
+		goto out;
+	if (!has_cache) {
+		spin_lock(&p->lock);
+		swap_entry_range_free(p, entry, nr);
+		spin_unlock(&p->lock);
+	}
+	*any_only_cache = has_cache;
+out:
+	return can_batch;
+}
+
 /*
  * Drop the last HAS_CACHE flag of swap entries, caller have to
  * ensure all entries belong to the same cgroup.
@@ -1797,6 +1849,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
 	bool any_only_cache = false;
 	unsigned long offset;
 	unsigned char count;
+	bool batched;
 
 	if (non_swap_entry(entry))
 		return;
@@ -1808,6 +1861,13 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
 	if (WARN_ON(end_offset > si->max))
 		goto out;
 
+	if (nr > 1 && swap_count(data_race(si->swap_map[start_offset]) == 1)) {
+		batched = try_batch_swap_entries_free(si, entry, nr,
+						&any_only_cache);
+		if (batched)
+			goto reclaim;
+	}
+
 	/*
 	 * First free all entries in the range.
 	 */
@@ -1821,6 +1881,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
 		}
 	}
 
+reclaim:
 	/*
 	 * Short-circuit the below loop if none of the entries had their
 	 * reference drop to zero.
-- 
2.34.1