mlx4_core: Avoid recycling old FMR R_Keys too soon

When a FMR is unmapped, mlx4 resets the map count to 0, and clears the
upper part of the R_Key which is used as the sequence counter.

This poses a problem for RDS, which uses ib_fmr_unmap as a fence
operation.  RDS assumes that after issuing an unmap, the old R_Keys
will be invalid for a "reasonable" period of time. For instance,
Oracle processes uses shared memory buffers allocated from a pool of
buffers.  When a process dies, we want to reclaim these buffers -- but
we must make sure there are no pending RDMA operations to/from those
buffers.  The only way to achieve that is by using unmap and sync the
TPT.

However, when the sequence count is reset on unmap, there is a high
likelihood that a new mapping will be given the same R_Key that was
issued a few milliseconds ago.

To prevent this, don't reset the sequence count when unmapping a FMR.

Signed-off-by: Olaf Kirch <olaf.kirch@oracle.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/net/mlx4/mr.c

@@ -607,15 +607,9 @@ EXPORT_SYMBOL_GPL(mlx4_fmr_enable);
 void mlx4_fmr_unmap(struct mlx4_dev *dev, struct mlx4_fmr *fmr,
 		    u32 *lkey, u32 *rkey)
 {
-	u32 key;
-
 	if (!fmr->maps)
 		return;
 
-	key = key_to_hw_index(fmr->mr.key);
-	key &= dev->caps.num_mpts - 1;
-	*lkey = *rkey = fmr->mr.key = hw_index_to_key(key);
-
 	fmr->maps = 0;
 
 	*(u8 *) fmr->mpt = MLX4_MPT_STATUS_SW;