58 lines
2.0 KiB
Diff
58 lines
2.0 KiB
Diff
From b31595992e52f94bd87c122dfad9cc37f15f82dc Mon Sep 17 00:00:00 2001
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From: Ben Hutchings <ben@decadent.org.uk>
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Date: Sun, 28 Nov 2010 23:46:46 +0000
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Subject: [PATCH] dm: Deal with merge_bvec_fn in component devices better
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This is analogous to commit 627a2d3c29427637f4c5d31ccc7fcbd8d312cd71,
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which does the same for md-devices at the top of the stack. The
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following explanation is taken from that commit. Thanks to Neil Brown
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<neilb@suse.de> for the advice.
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If a component device has a merge_bvec_fn then as we never call it
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we must ensure we never need to. Currently this is done by setting
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max_sector to 1 PAGE, however this does not stop a bio being created
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with several sub-page iovecs that would violate the merge_bvec_fn.
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So instead set max_segments to 1 and set the segment boundary to the
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same as a page boundary to ensure there is only ever one single-page
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segment of IO requested at a time.
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This can particularly be an issue when 'xen' is used as it is
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known to submit multiple small buffers in a single bio.
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Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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---
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drivers/md/dm-table.c | 14 +++++++-------
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1 files changed, 7 insertions(+), 7 deletions(-)
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diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
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index 90267f8..776734d 100644
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--- a/drivers/md/dm-table.c
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+++ b/drivers/md/dm-table.c
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@@ -511,15 +511,15 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
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(unsigned long long) start << SECTOR_SHIFT);
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/*
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- * Check if merge fn is supported.
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- * If not we'll force DM to use PAGE_SIZE or
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- * smaller I/O, just to be safe.
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+ * If we don't call merge_bvec_fn, we must never risk
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+ * violating it, so limit max_phys_segments to 1 lying within
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+ * a single page.
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*/
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+ if (q->merge_bvec_fn && !ti->type->merge) {
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+ limits->max_segments = 1;
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+ limits->seg_boundary_mask = PAGE_CACHE_SIZE - 1;
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+ }
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- if (q->merge_bvec_fn && !ti->type->merge)
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- limits->max_sectors =
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- min_not_zero(limits->max_sectors,
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- (unsigned int) (PAGE_SIZE >> 9));
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return 0;
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}
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EXPORT_SYMBOL_GPL(dm_set_device_limits);
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--
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1.7.2.3
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