Re: [PATCH v4 3/5] selftests/mm: fail KSM helpers on short reads and writes

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

On Sun, Nov 08, 2026 at 08:22:52PM -0500, Qiliang Yuan wrote:
> The KSM helpers in vm_util read and write KSM's sysfs and procfs files
> with a single pread() or write(), and return -errno when the call does
> not transfer what they asked for.
> 
> A read of zero bytes or a short write leaves errno at 0, so the helpers
> then return 0, which their callers take as a value of 0 or as a write
> that took effect. A short write of run makes ksm_start() wait for scans
> of a ksmd that never started, a short read of full_scans makes it wait
> for scans that never seem to complete, and short reads of
> ksm_merging_pages or ksm_stat and a short write of use_zero_pages make
> cases fail that should have been skipped.
> 
> Return -errno only when the call fails, and -EIO when it transfers less
> than asked, so that callers skip or fail as they do on other errors.
> Forcing each short read or write with an LD_PRELOAD shim on
> ksm_functional_tests:
> 
>   short read or write of   before                    after
>   run                      cases fail, then hangs    all skipped
>   full_scans               hangs in the first case   all skipped
>   ksm_merging_pages        9 of 11 cases fail        all skipped
>   ksm_stat                 zero page case fails      zero page case skipped
>   use_zero_pages           zero page case fails      zero page case skipped
> 
> Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
> ---
> The checks come from 93fb70aa5904 ("selftests/vm: add KSM unmerge
> tests"), 3d0745e59c84 ("selftest: add a testcase of ksm zero pages")
> and 42096aa24b82 ("selftest/mm: ksm_functional_tests: test in
> mmap_and_merge_range() if anything got merged"), and b27f292de6b1
> ("selftests/mm: put general ksm operation into vm_util") turned the
> use_zero_pages check into one of them. There are no Fixes tags, as
> only short reads and writes forced with the shim hit them, and the
> helpers are in vm_util only since v6.18.
> ---
>  tools/testing/selftests/mm/vm_util.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)

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

> 
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index 515031b8283da..caea045fa7d46 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -920,6 +920,8 @@ long ksm_get_self_zero_pages(void)
>  	close(proc_self_ksm_stat_fd);
>  	if (read_size < 0)
>  		return -errno;
> +	if (!read_size)
> +		return -EIO;
>  
>  	buf[read_size] = 0;
>  
> @@ -944,8 +946,10 @@ long ksm_get_self_merging_pages(void)
>  
>  	ret = pread(proc_self_ksm_merging_pages_fd, buf, sizeof(buf) - 1, 0);
>  	close(proc_self_ksm_merging_pages_fd);
> -	if (ret <= 0)
> +	if (ret < 0)
>  		return -errno;
> +	if (!ret)
> +		return -EIO;
>  	buf[ret] = 0;
>  
>  	return strtol(buf, NULL, 10);
> @@ -963,8 +967,10 @@ long ksm_get_full_scans(void)
>  
>  	ret = pread(ksm_full_scans_fd, buf, sizeof(buf) - 1, 0);
>  	close(ksm_full_scans_fd);
> -	if (ret <= 0)
> +	if (ret < 0)
>  		return -errno;
> +	if (!ret)
> +		return -EIO;
>  	buf[ret] = 0;
>  
>  	return strtol(buf, NULL, 10);
> @@ -981,7 +987,9 @@ int ksm_use_zero_pages(void)
>  
>  	ret = write(ksm_use_zero_pages_fd, "1", 1);
>  	close(ksm_use_zero_pages_fd);
> -	return ret == 1 ? 0 : -errno;
> +	if (ret < 0)
> +		return -errno;
> +	return ret == 1 ? 0 : -EIO;
>  }
>  
>  int ksm_start(void)
> @@ -1002,8 +1010,10 @@ int ksm_start(void)
>  	}
>  	ret = write(ksm_fd, "1", 1);
>  	close(ksm_fd);
> -	if (ret != 1)
> +	if (ret < 0)
>  		return -errno;
> +	if (ret != 1)
> +		return -EIO;
>  	do {
>  		end_scans = ksm_get_full_scans();
>  		if (end_scans < 0)
> @@ -1024,7 +1034,9 @@ int ksm_stop(void)
>  
>  	ret = write(ksm_fd, "2", 1);
>  	close(ksm_fd);
> -	return ret == 1 ? 0 : -errno;
> +	if (ret < 0)
> +		return -errno;
> +	return ret == 1 ? 0 : -EIO;
>  }
>  
>  int get_hardware_corrupted_size(unsigned long *val)
> 
> -- 
> 2.43.0
>