pfSense

This comes from a deficiency in the API exposed to userspace.

Consider the following:

tmpfs on /tmp (tmpfs, local)
/dev/md0 on /tmp (ufs, local)
tmpfs on /tmp/test (tmpfs, local)

df sorts this out in 2 phases:
1. mntsize = getmntinfo(&mntbuf, MNT_NOWAIT);

This gets ALL filesystems, but the MNT_NOWAIT flag tells the kernel to not issue any I/O.

2. mntsize = regetmntinfo(&mntbuf, mntsize, vfslist);

This walks the list and issues an additional statfs(2) for filesystems of interest by passing the mount point as an identifier.

In the example above there are 2 filesystems with the same mount point and the kernel exports the second one. Should there be 3 filesystems stacked up, there would be no way to ask for the middle one.

Thus if you filter for tmpfs, you end up getting ufs as the first result.

In other words the interface needs to be fixed.

In the meantime as a hack the above can be reworked -- getmntinfo can be called with MNT_WAIT and then the result filtered out. The downside which makes it not committable upstream is the updates requested from all mount points, even ones of no interest.

Mount points carry "fs id" which in principle could be filtered on, but getting the value requires privileges and there is still no syscall to filter on it.

Another work around would be to zfs set mountpoint=none on the problematic dataset.

The following is the hack which does the trick for me fwiw:

diff --git a/bin/df/df.c b/bin/df/df.c
index a0d43fca5789..1a11fae848f2 100644
--- a/bin/df/df.c
+++ b/bin/df/df.c
@@ -88,7 +88,7 @@ static void     prthuman(const struct statfs *, int64_t);
 static void      prthumanval(const char *, int64_t);
 static intmax_t          fsbtoblk(int64_t, uint64_t, u_long);
 static void      prtstat(struct statfs *, struct maxwidths *);
-static size_t    regetmntinfo(struct statfs **, long, const char **);
+static size_t    filtermntinfo(struct statfs **, long, const char **);
 static void      update_maxwidths(struct maxwidths *, const struct statfs *);
 static void      usage(void);

@@ -213,8 +213,8 @@ main(int argc, char *argv[])
        rv = 0;
        if (!*argv) {
                /* everything (modulo -t) */
-               mntsize = getmntinfo(&mntbuf, MNT_NOWAIT);
-               mntsize = regetmntinfo(&mntbuf, mntsize, vfslist);
+               mntsize = getmntinfo(&mntbuf, MNT_WAIT);
+               mntsize = filtermntinfo(&mntbuf, mntsize, vfslist);
        } else {
                /* just the filesystems specified on the command line */
                mntbuf = malloc(argc * sizeof(*mntbuf));
@@ -316,35 +316,24 @@ getmntpt(const char *name)
  * current (not cached) info.  Returns the new count of valid statfs bufs.
  */
 static size_t
-regetmntinfo(struct statfs **mntbufp, long mntsize, const char **vfslist)
+filtermntinfo(struct statfs **mntbufp, long mntsize, const char **vfslist)
 {
-       int error, i, j;
+       int i;
        struct statfs *mntbuf;

        if (vfslist == NULL)
                return (nflag ? mntsize : getmntinfo(mntbufp, MNT_WAIT));

        mntbuf = *mntbufp;
-       for (j = 0, i = 0; i < mntsize; i++) {
-               if (checkvfsname(mntbuf[i].f_fstypename, vfslist))
+
+       for (i = 0; i < mntsize; i++) {
+               if (checkvfsname(mntbuf[i].f_fstypename, vfslist)) {
+                       mntbuf[i].f_flags |= MNT_IGNORE;
                        continue;
-               /*
-                * XXX statfs(2) can fail for various reasons. It may be
-                * possible that the user does not have access to the
-                * pathname, if this happens, we will fall back on
-                * "stale" filesystem statistics.
-                */
-               error = statfs(mntbuf[i].f_mntonname, &mntbuf[j]);
-               if (nflag || error < 0)
-                       if (i != j) {
-                               if (error < 0)
-                                       xo_warnx("%s stats possibly stale",
-                                           mntbuf[i].f_mntonname);
-                               mntbuf[j] = mntbuf[i];
-                       }
-               j++;
+               }
        }
-       return (j);
+
+       return (mntsize);
 }

 static void