[PATCH v2] fstests: btrfs: Add regression test to check if btrfs can handle high devid

[Qu Wenruo <wqu@suse.com>]

Add a regression test to check if btrfs can handle high devid.

The test will add and remove devices to a btrfs fs, so that the devid
will increase to uncommon but still valid values.

The regression is introduced by kernel commit ab4ba2e13346 ("btrfs:
tree-checker: Verify dev item").
The fix is titled "btrfs: tree-checker: Fix wrong check on max devid".

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
Changelog:
v2:
- Small comment refinement
- Add more comment explaining some details, including:
  * Why node size affects the runtime
  * How the triggering threshold is calculated
  * Why the intermediate number 64 is used as iteration number
---
 tests/btrfs/194     | 86 +++++++++++++++++++++++++++++++++++++++++++++
 tests/btrfs/194.out |  2 ++
 tests/btrfs/group   |  1 +
 3 files changed, 89 insertions(+)
 create mode 100755 tests/btrfs/194
 create mode 100644 tests/btrfs/194.out

diff --git a/tests/btrfs/194 b/tests/btrfs/194
new file mode 100755
index 00000000..b7249f0d
--- /dev/null
+++ b/tests/btrfs/194
@@ -0,0 +1,86 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2019 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test 194
+#
+# Test if btrfs can handle large device ids.
+#
+# The regression is introduced by kernel commit ab4ba2e13346 ("btrfs:
+# tree-checker: Verify dev item").
+# The fix is titled: "btrfs: tree-checker: Fix wrong check on max devid"
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs btrfs
+_supported_os Linux
+_require_scratch_dev_pool 2
+_scratch_dev_pool_get 2
+
+# Here we use 4k node size to reduce runtime (explained near _scratch_mkfs call)
+# To use the minimal node size (4k) we need 4K page size.
+if [ $(get_page_size) != 4096 ]; then
+	_notrun "This test need 4k page size"
+fi
+
+device_1=$(echo $SCRATCH_DEV_POOL | awk '{print $1}')
+device_2=$(echo $SCRATCH_DEV_POOL | awk '{print $2}')
+
+echo device_1=$device_1 device_2=$device_2 >> $seqres.full
+
+# The wrong check limit is based on the max item size (BTRFS_MAX_DEVS() macro),
+# and max item size is based on node size, so smaller node size will result
+# much shorter runtime. So here we use minimal node size (4K) to reduce runtime.
+_scratch_mkfs -n 4k >> $seqres.full
+_scratch_mount
+
+# For 4k nodesize, the wrong limit is calculated by:
+# ((4096 - 101 - 25 - 80) / 32) + 1
+#    |      |    |    |     |- sizeof(btrfs_stripe)
+#    |      |    |    |- sizeof(btrfs_chunk)
+#    |      |    |- sizeof(btrfs_item)
+#    |      |- sizeof(btrfs_header)
+#    |- node size
+# Which is 122.
+#
+# The old limit is wrong because it doesn't take devid holes into consideration.
+# We can have large devid, but still have only 1 device.
+#
+# Add and remove device in a loop, each iteration will increase devid by 2.
+# So by 64 iterations, we will definitely hit that 122 limit.
+for (( i = 0; i < 64; i++ )); do
+	$BTRFS_UTIL_PROG device add -f $device_2 $SCRATCH_MNT
+	$BTRFS_UTIL_PROG device del $device_1 $SCRATCH_MNT
+	$BTRFS_UTIL_PROG device add -f $device_1 $SCRATCH_MNT
+	$BTRFS_UTIL_PROG device del $device_2 $SCRATCH_MNT
+done
+_scratch_dev_pool_put
+
+echo "Silence is golden"
+
+# success, all done
+status=0
+exit
diff --git a/tests/btrfs/194.out b/tests/btrfs/194.out
new file mode 100644
index 00000000..7bfd50ff
--- /dev/null
+++ b/tests/btrfs/194.out
@@ -0,0 +1,2 @@
+QA output created by 194
+Silence is golden
diff --git a/tests/btrfs/group b/tests/btrfs/group
index b92cb12c..d8aafe20 100644
--- a/tests/btrfs/group
+++ b/tests/btrfs/group
@@ -196,3 +196,4 @@
 191 auto quick send dedupe
 192 auto replay snapshot stress
 193 auto quick qgroup enospc limit
+194 auto volume
-- 
2.22.0