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xen-blkfront: correct maximum segment accounting

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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@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
This commit is contained in:
Jan Beulich 2017-01-23 08:11:37 -07:00 committed by Konrad Rzeszutek Wilk
parent b32728ffef
commit 3b4f18843e
1 changed files with 9 additions and 3 deletions
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12
drivers/block/xen-blkfront.c
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@ -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);