[PATCH v2 0/9] Optimize anonymous large folio unmapping

[Dev Jain]

Speed up unmapping of anonymous large folios by clearing the ptes, and
setting swap ptes, in one go.

The following benchmark (stolen from Barry at [1]) is used to measure the
time taken to swapout 256M worth of memory backed by 64K large folios:

 #define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/mman.h>
 #include <string.h>
 #include <time.h>
 #include <unistd.h>
 #include <errno.h>

 #define SIZE_MB 256
 #define SIZE_BYTES (SIZE_MB * 1024 * 1024)

 int main() {
     void *addr = mmap(NULL, SIZE_BYTES, PROT_READ | PROT_WRITE,
                       MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
     if (addr == MAP_FAILED) {
         perror("mmap failed");
         return 1;
     }

     memset(addr, 0, SIZE_BYTES);

     struct timespec start, end;
     clock_gettime(CLOCK_MONOTONIC, &start);

     if (madvise(addr, SIZE_BYTES, MADV_PAGEOUT) != 0) {
         perror("madvise(MADV_PAGEOUT) failed");
         munmap(addr, SIZE_BYTES);
         return 1;
     }

     clock_gettime(CLOCK_MONOTONIC, &end);

     long duration_ns = (end.tv_sec - start.tv_sec) * 1e9 +
                        (end.tv_nsec - start.tv_nsec);
     printf("madvise(MADV_PAGEOUT) took %ld ns (%.3f ms)\n",
            duration_ns, duration_ns / 1e6);

     munmap(addr, SIZE_BYTES);
     return 0;
 }

Performance as measured on a Linux VM on Apple M3 (arm64):

Vanilla - Mean: 37401913 ns, std dev: 12%
Patched - Mean: 17420282 ns, std dev: 11%

No regression observed on 4K folios.

Performance as measured on bare metal x86:

Vanilla - mean: 54986286 ns, std dev: 1.5%
Patched - mean: 51930795 ns, std dev: 3%

Interestingly, no obvious improvement is observed on x86, hinting that the
benefit lies mainly in the reduction of ptep_get() calls and the reduction
of TLB flushes during contpte-unfolding, on arm64.

No regression is observed on 4K folios on x86 too.

---
Based on mm-unstable 3fa44141e0bb ("ksm: optimize rmap_walk_ksm by passing
a suitable address range"). mm-selftests pass.

v1->v2:
 - Keep nr_pages as unsigned long
 - Add patch 2
 - Rename some functions, make return type bool for functions returning 0/1
 - Drop page_vma_mapped_walk_jump - this is implicitly handled
 - Drop likely()
 - Add folio_dup/put_swap_pages, do subpage -> page
 - Shorten the kerneldoc to remove unnecessary information - keep it
   aligned with analogous functions
 - Put clear_pages_anon_exclusive to mm.h
 - Some more refactoring in last patch with finish_folio_unmap

Dev Jain (9):
  mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one
  mm/rmap: refactor hugetlb pte clearing in try_to_unmap_one
  mm/rmap: refactor some code around lazyfree folio unmapping
  mm/memory: Batch set uffd-wp markers during zapping
  mm/rmap: batch unmap folios belonging to uffd-wp VMAs
  mm/swapfile: Add batched version of folio_dup_swap
  mm/swapfile: Add batched version of folio_put_swap
  mm/rmap: Add batched version of folio_try_share_anon_rmap_pte
  mm/rmap: enable batch unmapping of anonymous folios

 include/linux/mm.h        |  11 ++
 include/linux/mm_inline.h |  32 +--
 include/linux/rmap.h      |  27 ++-
 mm/internal.h             |  26 +++
 mm/memory.c               |  26 +--
 mm/mprotect.c             |  17 --
 mm/rmap.c                 | 404 +++++++++++++++++++++++---------------
 mm/shmem.c                |   8 +-
 mm/swap.h                 |  23 ++-
 mm/swapfile.c             |  42 ++--
 10 files changed, 380 insertions(+), 236 deletions(-)

-- 
2.34.1