[Bug 196405] mkdir mishandles st_nlink in ext4 directory with 64997 subdirectories

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=196405

--- Comment #6 from Paul Eggert (eggert@cs.ucla.edu) ---
(In reply to Andreas Dilger from comment #5)
> the "link count == 1" meaning is historical for ancient filesystems ...
> it didn't mean "lots of subdirectories", but rather "I don't know the
> subdirectory count at all".

This meaning is still true for ext4. For example:

$ mkdir d d/{1..64998}
$ rm -fr d
$ mkdir d
$ ls -ld d
drwxr-xr-x 2 eggert eggert 4096 Jul 18 15:14 d
$ mkdir d/{1..64998}
$ rmdir d/*
$ ls -ld d
drwxr-xr-x 1 eggert eggert 1441792 Jul 18 15:14 d

That last link count of 1 means "I don't know the subdirectory count", even
though d has no subdirectories whatsoever.

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