Re: [PATCH v4 4/5] selftests/mm: move waiting for KSM full scans into a helper

["Xu Xin (vivo)" <xu.xin@linux.dev>]

On Sun, Nov 08, 2026 at 08:22:53PM -0500, Qiliang Yuan wrote:
> ksm_start() enables ksmd and then polls full_scans until two more full
> scans have completed, so that any merging that can happen has happened.
> 
> There is no helper to wait for a number of full scans while ksmd is
> already running, so a test that needs one has to copy that loop.
> 
> Move the loop into ksm_wait_full_scans() and make ksm_start() call it
> after enabling ksmd. The count is now read after ksmd starts rather than
> before, which can only make ksm_start() wait longer.
> 
> Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
> Tested-by: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>

Reviewed-by: Xu Xin (vivo) <xu.xin@linux.dev>

> ---
>  tools/testing/selftests/mm/vm_util.c | 31 ++++++++++++++++++-------------
>  tools/testing/selftests/mm/vm_util.h |  1 +
>  2 files changed, 19 insertions(+), 13 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index caea045fa7d46..8184691211acc 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -992,35 +992,40 @@ int ksm_use_zero_pages(void)
>  	return ret == 1 ? 0 : -EIO;
>  }
>  
> +int ksm_wait_full_scans(long nr)
> +{
> +	long start_scans, end_scans;
> +
> +	start_scans = ksm_get_full_scans();
> +	if (start_scans < 0)
> +		return start_scans;
> +	do {
> +		end_scans = ksm_get_full_scans();
> +		if (end_scans < 0)
> +			return end_scans;
> +	} while (end_scans < start_scans + nr);
> +
> +	return 0;
> +}
> +
>  int ksm_start(void)
>  {
>  	int ksm_fd;
>  	ssize_t ret;
> -	long start_scans, end_scans;
>  
>  	ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR);
>  	if (ksm_fd < 0)
>  		return -errno;
>  
> -	/* Wait for two full scans such that any possible merging happened. */
> -	start_scans = ksm_get_full_scans();
> -	if (start_scans < 0) {
> -		close(ksm_fd);
> -		return start_scans;
> -	}
>  	ret = write(ksm_fd, "1", 1);
>  	close(ksm_fd);
>  	if (ret < 0)
>  		return -errno;
>  	if (ret != 1)
>  		return -EIO;
> -	do {
> -		end_scans = ksm_get_full_scans();
> -		if (end_scans < 0)
> -			return end_scans;
> -	} while (end_scans < start_scans + 2);
>  
> -	return 0;
> +	/* Wait for two full scans such that any possible merging happened. */
> +	return ksm_wait_full_scans(2);
>  }
>  
>  int ksm_stop(void)
> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> index b5d59729d4327..7e00030077d2f 100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -162,6 +162,7 @@ void *sys_mremap(void *old_address, unsigned long old_size,
>  long ksm_get_self_zero_pages(void);
>  long ksm_get_self_merging_pages(void);
>  long ksm_get_full_scans(void);
> +int ksm_wait_full_scans(long nr);
>  int ksm_use_zero_pages(void);
>  int ksm_start(void);
>  int ksm_stop(void);
> 
> -- 
> 2.43.0
>