By default, unless pnfs is used coalesce pages until pg_bsize
(rsize or wsize) is reached.
pnfs layout drivers define their own pg_test methods that use
pnfs_generic_pg_test and need to define their own I/O size
limits (e.g. based on the file stripe size).
[Move a check from nfs_pageio_do_add_request to nfs_generic_pg_test] Signed-off-by: Boaz Harrosh <bharrosh@panasas.com> Signed-off-by: Benny Halevy <bhalevy@panasas.com>