Re: [PATCH v1 3/3] selftests/mm: Speed up split_huge_page_test

[Rafael Aquini <raquini@redhat.com>]

On Tue, Mar 18, 2025 at 05:43:41PM +0000, Ryan Roberts wrote:
> create_pagecache_thp_and_fd() was previously writing a file sized at
> twice the PMD size by making a per-byte write syscall. This was quite
> slow when the PMD size is 4M, but completely intolerable for 32M (PMD
> size for arm64's 16K page size), and 512M (PMD size for arm64's 64K page
> size).
> 
> The byte pattern has a 256 byte period, so let's create a 1K buffer and
> fill it with exactly 4 periods. Then we can write the buffer as many
> times as is required to fill the file. This makes things much more
> tolerable.
> 
> The test now passes for 16K page size. It still fails for 64K page size
> because MAX_PAGECACHE_ORDER is too small for 512M folio size (I think).
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  tools/testing/selftests/mm/split_huge_page_test.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> index 3f353f3d070f..499333d75fff 100644
> --- a/tools/testing/selftests/mm/split_huge_page_test.c
> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> @@ -5,6 +5,7 @@
>   */
>  
>  #define _GNU_SOURCE
> +#include <assert.h>
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <stdarg.h>
> @@ -361,6 +362,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
>  {
>  	size_t i;
>  	int dummy = 0;
> +	unsigned char buf[1024];
>  
>  	srand(time(NULL));
>  
> @@ -368,11 +370,12 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
>  	if (*fd == -1)
>  		ksft_exit_fail_msg("Failed to create a file at %s\n", testfile);
>  
> -	for (i = 0; i < fd_size; i++) {
> -		unsigned char byte = (unsigned char)i;
> +	assert(fd_size % sizeof(buf) == 0);
> +	for (i = 0; i < sizeof(buf); i++)
> +		buf[i] = (unsigned char)i;
> +	for (i = 0; i < fd_size; i += sizeof(buf))
> +		write(*fd, buf, sizeof(buf));
>  
> -		write(*fd, &byte, sizeof(byte));
> -	}
>  	close(*fd);
>  	sync();
>  	*fd = open("/proc/sys/vm/drop_caches", O_WRONLY);
> -- 
> 2.43.0
> 
> 

Acked-by: Rafael Aquini <raquini@redhat.com>