On Fri, Sep 01, 2017 at 08:07:16AM -0700, Darrick J. Wong wrote:

fmh_entries is the number of records returned, not the number of records
in the dataset.  If, for example, you allocate space for 100 records and
perform a query for a block that has been reflinked 1000 times, the
dataset size is 1000 but fmh_entries is set to 100.  The lack of a LAST
flag on the 100th record tells you that there's more records to return.

If however you allocate space for 100 records and the block is reflinked
exactly 100 times, there's no way (without the flag) for userspace to
know that record 100 is the end of the dataset, so the only thing it can
do is to fsmap_advance() and try the query again, only to receive zero
results.  Granted I don't think fsmap queries are all /that/ expensive,
but it's trivial for the kernel to set the flag.

Oh, ok - it had to look up this defintion of dataset first.  With
that the explanation makes total sense:

Reviewed-by: Christoph Hellwig <hch@lst.de>