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Commit 3b4f1884 authored by Jan Beulich's avatar Jan Beulich Committed by Konrad Rzeszutek Wilk
Browse files

xen-blkfront: correct maximum segment accounting 



Making use of "max_indirect_segments=" has issues:
- blkfront_setup_indirect() may end up with zero psegs when PAGE_SIZE
  is sufficiently much larger than XEN_PAGE_SIZE
- the variable driven by the command line option
  (xen_blkif_max_segments) has a somewhat different purpose, and hence
  should namely never end up being zero
- as long as the specified value is lower than the legacy default,
  we better don't use indirect segments at all (or we'd in fact lower
  throughput)

Signed-off-by: default avatarJan Beulich <jbeulich@suse.com>
Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
parent b32728ff

drivers/block/xen-blkfront.c

+9 −3
Original line number Diff line number Diff line
@@ -2223,7 +2223,7 @@ static int blkfront_setup_indirect(struct blkfront_ring_info *rinfo) 
	}

	else

		grants = info->max_indirect_segments;

	psegs = grants / GRANTS_PER_PSEG;

	psegs = DIV_ROUND_UP(grants, GRANTS_PER_PSEG);



	err = fill_grant_buffer(rinfo,

				(grants + INDIRECT_GREFS(grants)) * BLK_RING_SIZE(info));
@@ -2328,8 +2328,11 @@ static void blkfront_gather_backend_features(struct blkfront_info *info) 


	indirect_segments = xenbus_read_unsigned(info->xbdev->otherend,

					"feature-max-indirect-segments", 0);

	info->max_indirect_segments = min(indirect_segments,

					  xen_blkif_max_segments);

	if (indirect_segments > xen_blkif_max_segments)

		indirect_segments = xen_blkif_max_segments;

	if (indirect_segments <= BLKIF_MAX_SEGMENTS_PER_REQUEST)

		indirect_segments = 0;

	info->max_indirect_segments = indirect_segments;

}



/*
@@ -2652,6 +2655,9 @@ static int __init xlblk_init(void) 
	if (!xen_domain())

		return -ENODEV;



	if (xen_blkif_max_segments < BLKIF_MAX_SEGMENTS_PER_REQUEST)

		xen_blkif_max_segments = BLKIF_MAX_SEGMENTS_PER_REQUEST;



	if (xen_blkif_max_ring_order > XENBUS_MAX_RING_GRANT_ORDER) {

		pr_info("Invalid max_ring_order (%d), will use default max: %d.\n",

			xen_blkif_max_ring_order, XENBUS_MAX_RING_GRANT_ORDER);