Re: [PATCH] nfsrahead: enable event-driven mountinfo monitoring

[Steve Dickson <steved@redhat.com>]

On 2/11/26 2:01 PM, Aaron Tomlin wrote:
> The nfsrahead utility relies on parsing "/proc/self/mountinfo" to
> correlate a device number with a specific NFS mount point. However, due
> to the asynchronous nature of system initialisation, the relevant entry
> in mountinfo may not be immediately available when the tool is executed.
> 
> Currently, the utility employs a naive polling mechanism, retrying the
> search five times with a fixed 50ms delay (totalling 250ms). This
> approach proves brittle on systems under high load or during
> distinctively slow boot sequences, where the population of the mount
> table may exceed this brief window. Consequently, nfsrahead fails to
> configure the readahead value.
> 
> To mitigate this race condition and improve robustness, update
> get_device_info() to utilise the libmount monitoring API.
> 
> The new implementation:
> 
>      1.	Initialises a monitor on /proc/self/mountinfo using
> 	mnt_new_monitor().
> 
>      2.	Replaces the fixed polling loop with mnt_monitor_wait(),
> 	allowing the process to sleep until the Linux kernel notifies
> 	userspace of a change to the mount table.
> 
>      3.	Increases the maximum wait time to 10 seconds (MNT_NM_TIMEOUT),
> 	significantly reducing the likelihood of a timeout failure
> 	whilst ensuring the tool returns immediately once the mount
> 	appears.
> 
>      4.	Retains the original polling logic as a fallback mechanism
> 	should the monitor fail to initialise.
> 
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Committed... (tag: nfs-utils-2-8-6-rc2)

steved.
> ---
>   tools/nfsrahead/main.c | 35 ++++++++++++++++++++++++++++++++++-
>   1 file changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/nfsrahead/main.c b/tools/nfsrahead/main.c
> index b7b889ff..64953346 100644
> --- a/tools/nfsrahead/main.c
> +++ b/tools/nfsrahead/main.c
> @@ -16,6 +16,7 @@
>   
>   #define CONF_NAME "nfsrahead"
>   #define NFS_DEFAULT_READAHEAD 128
> +#define MNT_NM_TIMEOUT 10000
>   
>   /* Device information from the system */
>   struct device_info {
> @@ -117,7 +118,39 @@ out_free_device_info:
>   
>   static int get_device_info(const char *device_number, struct device_info *device_info)
>   {
> -	int ret = get_mountinfo(device_number, device_info, MOUNTINFO_PATH);
> +	int ret;
> +	struct libmnt_monitor *mn = NULL;
> +	int timeout_ms = MNT_NM_TIMEOUT;
> +
> +	ret = get_mountinfo(device_number, device_info, MOUNTINFO_PATH);
> +	if (ret == 0)
> +		return 0;
> +
> +	mn = mnt_new_monitor();
> +	if (!mn)
> +		goto fallback;
> +
> +	if (mnt_monitor_enable_kernel(mn, 1) < 0) {
> +		mnt_unref_monitor(mn);
> +		goto fallback;
> +	}
> +
> +	while (timeout_ms > 0) {
> +		int rc = mnt_monitor_wait(mn, timeout_ms);
> +		if (rc > 0) {
> +			ret = get_mountinfo(device_number, device_info, MOUNTINFO_PATH);
> +			if (ret == 0) {
> +				mnt_unref_monitor(mn);
> +				return 0;
> +			}
> +		} else {
> +			break;
> +		}
> +	}
> +	mnt_unref_monitor(mn);
> +	return ret;
> +
> +fallback:
>   	for (int retry_count = 0; retry_count < 5 && ret != 0; retry_count++) {
>   		usleep(50000);
>   		ret = get_mountinfo(device_number, device_info, MOUNTINFO_PATH);