Quoting myself from #musl:
01:59:40 hm, I think this was just invented for symmetry with bfree/bavail
02:00:46 FFS has minfree for space but nothing equivalent for inodes
02:32:31 (this is mirrored in ext4;
	  a global grep over DragonFlyBSD and the illumos gate
	  showed just NFSv3 forwarding from the server;
	  OpenBSD always sets it to the same thing as f_ffree;
	  oddly, NetBSD /does/ calculate it differently
	  for LFS and FFS but due to queued writes or
	  w/e not because of root reservation;
	  and as expected a lot of "/* what to put in here? */"
	                       and "// XXX same??")

Link: https://lore.kernel.org/linux-man/f54kudgblgk643u32tb6at4cd3kkzha6hslahv24szs4raroaz@ogivjbfdaqtb/t/#u
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
---
 man3/statvfs.3 | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/man3/statvfs.3 b/man3/statvfs.3
index b1f0e7545..272ee5391 100644
--- a/man3/statvfs.3
+++ b/man3/statvfs.3
@@ -227,6 +227,14 @@ .SH NOTES
 .BR statvfs ()
 with the argument
 .IR path .
+.PP
+Under Linux,
+.I f_favail
+is always the same as
+.IR f_ffree ,
+and there's no way for a file-system to report otherwise.
+This is not an issue, since no filesystems with an i-node
+root reservation exist.
 .SH STANDARDS
 POSIX.1-2008.
 .SH HISTORY
-- 
2.39.2