[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 #4 from Andreas Dilger (adilger.kernelbugzilla@dilger.ca) ---
The limit is implemented as < 65000, which was somewhat below the hard limit of
the ext4_inode.i_links_count __u16 field of 65535.  There is no hard reason for
65000 except to allow some margin for detecting overflow, and reserved values
if there was a need.  I don't think it makes a big practical difference whether
directory link counts are accurate up to 65000 or 65534 subdirectories, since
very few systems have this many subdirectories.

In theory, we could add an i_links_count_hi field to extend this to a 32-bit
value, but it isn't clear if there is a big benefit from doing this?

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