On Tue, Feb 07, 2017 at 01:34:54PM -0500, Keith Busch wrote:

On Tue, Feb 07, 2017 at 05:46:58PM +0100, Christoph Hellwig wrote:

quoted

@@ -1233,6 +1243,8 @@ static void nvme_set_queue_limits(struct nvme_ctrl *ctrl,
 	if (ctrl->vwc & NVME_CTRL_VWC_PRESENT)
 		vwc = true;
 	blk_queue_write_cache(q, vwc, vwc);
+	blk_queue_max_discard_segments(q,
+			PAGE_SIZE / sizeof(struct nvme_dsm_range));

We need to use ctrl->page_size rather than PAGE_SIZE.

We need the PAGE_SIZE upper bound because the "special" bvec only
allows a single page payload.  I can't find any indication that
the DSM command is limited to a single PRP, so not sure where a
ctrl->page_size would come from.  But now that I looked again there
is a hard limit of 256 entries due to NR being a 8-bit field, so
I need to cap on that at least.