[Xen-devel] [PATCH] xen-blkfront: correct maximum segment accounting

["Jan Beulich" <JBeulich@xxxxxxxx>]

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: Jan Beulich <jbeulich@xxxxxxxx>
---
 drivers/block/xen-blkfront.c |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

--- 4.10-rc5/drivers/block/xen-blkfront.c
+++ 4.10-rc5-xen-blkfront-seg-accounting/drivers/block/xen-blkfront.c
@@ -2223,7 +2223,7 @@ static int blkfront_setup_indirect(struc
        }
        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_feat
 
        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);




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