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

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

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/

Link: https://lkml.kernel.org/r/20240807215859.57491-3-21cnbao@gmail.com
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Acked-by: David Hildenbrand <david@redhat.com>
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>