Re: [PATCH 0/9] mm/rmap: Optimize anonymous large folio unmapping

[Lorenzo Stoakes (Oracle)]

On Tue, Mar 10, 2026 at 01:00:04PM +0530, Dev Jain wrote:
> 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;
>  }
>
> On arm64, only showing one of the middle values in the distribution:
>

This doesn't seem very statistically valid.

How about you give median, stddev etc.? Variance matters too.

> without patch:
> madvise(MADV_PAGEOUT) took 52192959 ns (52.193 ms)
>
> with patch:
> madvise(MADV_PAGEOUT) took 26676625 ns (26.677 ms)

You have a habit of only giving data on arm64, and not mentioning whether you've
tested on any other arch/setup.

I've commented on this before so I'm a bit disappointed you've done the exact
same thing here again. Especially since you've previously introduced regressions
this way.

Please can you test this on (hardware!) x86-64 _at least_ as well and confirm
you aren't regressing anything for 4K pages?

>
>
> [1] https://lore.kernel.org/all/20250513084620.58231-1-21cnbao@xxxxxxxxx/
>
> ---
> Based on mm-unstable bb420884e9e0. mm-selftests pass.
>
> Dev Jain (9):
>   mm/rmap: make nr_pages signed in try_to_unmap_one
>   mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one
>   mm/rmap: refactor lazyfree unmap commit path to
>     commit_ttu_lazyfree_folio()
>   mm/memory: Batch set uffd-wp markers during zapping
>   mm/rmap: batch unmap folios belonging to uffd-wp VMAs
>   mm/swapfile: Make folio_dup_swap batchable
>   mm/swapfile: Make folio_put_swap batchable
>   mm/rmap: introduce folio_try_share_anon_rmap_ptes
>   mm/rmap: enable batch unmapping of anonymous folios
>
>  include/linux/mm_inline.h  |  37 +++--
>  include/linux/page-flags.h |  11 ++
>  include/linux/rmap.h       |  38 ++++-
>  mm/internal.h              |  26 ++++
>  mm/memory.c                |  26 +---
>  mm/mprotect.c              |  17 ---
>  mm/rmap.c                  | 274 ++++++++++++++++++++++++-------------
>  mm/shmem.c                 |   8 +-
>  mm/swap.h                  |  10 +-
>  mm/swapfile.c              |  25 ++--
>  10 files changed, 298 insertions(+), 174 deletions(-)
>
> --
> 2.34.1
>