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

[Dev Jain <dev.jain@arm.com>]

On 10/03/26 1:32 pm, Lorenzo Stoakes (Oracle) wrote:
> 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.

Okay.

> 
>> 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 did do an x86 build, forgot to mention that.
I didn't do the numbers thinking this patchset is quite generic and has got
nothing to do with the arm64 cont bit - but arguably I should have.

> 
> 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?

Lemme go and manage that :)

> 
>>
>>
>> [1] https://lore.kernel.org/all/20250513084620.58231-1-21cnbao@gmail.com/
>>
>> ---
>> 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
>>