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@xxxxxx>
