Re: [PATCH] nfsrahead: enable event-driven mountinfo monitoring and skip non-NFS devices

[Steve Dickson <steved@redhat.com>]

On 3/6/26 11:19 AM, 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 heavy load or during
> distinctively slow boot sequences.
> 
> To mitigate this race condition and improve robustness, update
> get_device_info() to utilise the libmount monitoring API.
> 
> The new implementation introduces the following logic:
> 
>      1.  Initialises a monitor on /proc/self/mountinfo using
>          mnt_new_monitor().
> 
>      2.  Replaces the fixed polling loop with mnt_monitor_wait().
> 
>      3.  Increases the maximum wait time to 10 seconds (MNT_NM_TIMEOUT).
> 
>      4.  Introduces a fast-path rejection mechanism. NFS backing devices are
>          allocated from the kernel's unnamed block device pool (major number
>          0). While some local multi-device filesystems (such as Btrfs) also
>          utilise anonymous device numbers, physical hardware block devices
>          (e.g., sda, nvme) always possess specific, non-zero major numbers.
>          By instantly exiting with -ENODEV for any device string not
>          beginning with "0:", we safely bypass the monitor for physical
>          drives, preventing the exhaustion of udev worker threads.
>          See set_anon_super() and get_anon_bdev().
> 
>      5.  Implements strict monotonic deadline tracking within the monitor
>          loop to prevent indefinite blocking.
> 
> Fixes: 2b62ac4c ("nfsrahead: enable event-driven mountinfo monitoring")
> Reported-by: Yi Zhang <yi.zhang@redhat.com>
> Link: https://lore.kernel.org/linux-block/CAHj4cs8URj2fJ7KyP9ViAm6npVOaMiAErnw2uFyPYEU2wb7G_w@mail.gmail.com/T/#t
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Committed... (tag: nfs-utils-2-8-6-rc4)

steved.
> ---
> 
> Hi Steve,
> 
> This patch should resolve the udev worker exhaustion issue reported by
> Yi. It applies cleanly on top of the current nfs-utils tree, after your
> revert [1].
> 
> Thank you.
> 
> [1]: https://lore.kernel.org/linux-nfs/20260305124221.55407-1-steved@redhat.com/
> 
> 
>   tools/nfsrahead/main.c | 55 +++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 54 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/nfsrahead/main.c b/tools/nfsrahead/main.c
> index b7b889ff..78cd2581 100644
> --- a/tools/nfsrahead/main.c
> +++ b/tools/nfsrahead/main.c
> @@ -3,6 +3,7 @@
>   #include <stdlib.h>
>   #include <errno.h>
>   #include <unistd.h>
> +#include <time.h>
>   
>   #include <libmount/libmount.h>
>   #include <sys/sysmacros.h>
> @@ -17,6 +18,8 @@
>   #define CONF_NAME "nfsrahead"
>   #define NFS_DEFAULT_READAHEAD 128
>   
> +#define MNT_NM_TIMEOUT 10000
> +
>   /* Device information from the system */
>   struct device_info {
>   	char *device_number;
> @@ -117,7 +120,57 @@ 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;
> +	struct timespec start, now;
> +	int remaining_ms = MNT_NM_TIMEOUT;
> +
> +	/*
> +	 * Fast-path rejection:
> +	 * NFS backing devices always use the anonymous block device major number (0).
> +	 * If the device number does not start with "0:", it is a physical block device
> +	 * and will never be an NFS mount. Exit immediately to prevent blocking udev.
> +	 */
> +	if (strncmp(device_number, "0:", 2) != 0)
> +		return -ENODEV;
> +
> +	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;
> +	}
> +
> +	clock_gettime(CLOCK_MONOTONIC, &start);
> +
> +	while (remaining_ms > 0) {
> +		int rc = mnt_monitor_wait(mn, remaining_ms);
> +		if (rc > 0) {
> +			ret = get_mountinfo(device_number, device_info, MOUNTINFO_PATH);
> +			if (ret == 0) {
> +				mnt_unref_monitor(mn);
> +				return 0;
> +			}
> +		} else {
> +			break;
> +		}
> +
> +		clock_gettime(CLOCK_MONOTONIC, &now);
> +		long elapsed_ms = (now.tv_sec - start.tv_sec) * 1000 +
> +				  (now.tv_nsec - start.tv_nsec) / 1000000;
> +		remaining_ms = MNT_NM_TIMEOUT - elapsed_ms;
> +	}
> +
> +	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);