Re: [PATCH] generic/558: limit the number of spawned subprocesses

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

On Tue, Jul 11, 2023 at 05:51:42PM +0200, Mikulas Patocka wrote:
> When I run the test 558 on bcachefs, it works like a fork-bomb and kills
> the machine. The reason is that the "while" loop spawns "create_file"
> subprocesses faster than they are able to complete.
> 
> This patch fixes the crash by limiting the number of subprocesses to 128.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  tests/generic/558 |    1 +
>  1 file changed, 1 insertion(+)
> 
> Index: xfstests-dev/tests/generic/558
> ===================================================================
> --- xfstests-dev.orig/tests/generic/558
> +++ xfstests-dev/tests/generic/558
> @@ -48,6 +48,7 @@ echo "Create $((loop * file_per_dir)) fi
>  while [ $i -lt $loop ]; do
>  	create_file $SCRATCH_MNT/testdir $file_per_dir $i >>$seqres.full 2>&1 &
>  	let i=$i+1
> +	if [ $((i % 128)) = 0 ]; then wait; fi

Hm.  $loop is (roughly) the number of free inodes divided by 1000.  This
test completes nearly instantly on XFS; how many free inodes does
bcachefs report after _scratch_mount?

XFS reports ~570k inodes, so it's "only" starting 570 processes.

I think it's probably wise to clamp $loop to something sane, but let's
get to the bottom of how the math went wrong and we got a forkbomb.

--D

>  done
>  wait
>  
>