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

[Lance Yang]

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

Good numbers! Just tested on x86 KVM with THP=never, no performance
regression observed.

Cheers,
Lance