[PATCH] generic/558: avoid forkbombs on filesystems with many free inodes

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

Mikulas reported that this test became a forkbomb on his system when he
tested it with bcachefs.  Unlike XFS and ext4, which have large inodes
consuming hundreds of bytes, bcachefs has very tiny ones.  Therefore, it
reports a large number of free inodes on a freshly mounted 1GB fs (~15
million), which causes this test to try to create 15000 processes.

There's really no reason to do that -- all this test wanted to do was to
exhaust the number of inodes as quickly as possible using all available
CPUs, and then it ran xfs_repair to try to reproduce a bug.  Set the
number of subshells to 4x the CPU count and spread the work among
them instead of forking thousands of processes.

Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Mikulas Patocka <mpatocka@redhat.com>
---
 tests/generic/558 |   18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/tests/generic/558 b/tests/generic/558
index 4e22ce656b..de5c28d00d 100755
--- a/tests/generic/558
+++ b/tests/generic/558
@@ -39,15 +39,21 @@ _scratch_mkfs_sized $((1024 * 1024 * 1024)) >>$seqres.full 2>&1
 _scratch_mount
 
 i=0
-free_inode=`_get_free_inode $SCRATCH_MNT`
-file_per_dir=1000
-loop=$((free_inode / file_per_dir + 1))
+free_inodes=$(_get_free_inode $SCRATCH_MNT)
+nr_cpus=$(( $($here/src/feature -o) * 4 * LOAD_FACTOR ))
+echo "free inodes: $free_inodes nr_cpus: $nr_cpus" >> $seqres.full
+
+if ((free_inodes <= nr_cpus)); then
+	nr_cpus=1
+	files_per_dir=$free_inodes
+else
+	files_per_dir=$(( (free_inodes + nr_cpus - 1) / nr_cpus ))
+fi
 mkdir -p $SCRATCH_MNT/testdir
 
 echo "Create $((loop * file_per_dir)) files in $SCRATCH_MNT/testdir" >>$seqres.full
-while [ $i -lt $loop ]; do
-	create_file $SCRATCH_MNT/testdir $file_per_dir $i >>$seqres.full 2>&1 &
-	let i=$i+1
+for ((i = 0; i < nr_cpus; i++)); do
+	create_file $SCRATCH_MNT/testdir $files_per_dir $i >>$seqres.full 2>&1 &
 done
 wait