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

[Mikulas Patocka <mpatocka@redhat.com>]

On Tue, 11 Jul 2023, Darrick J. Wong wrote:

> 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

bcachefs reports 14509106 total inodes (for a 1GB filesystem)

As the test proceeds, the number of total inodes (as well as the number of 
free inodes) decreases.

Mikulas