On Tue, Jul 11, 2023 at 04:44:39PM -0700, 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@xxxxxxxxxx> > > > > --- > > 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. It's because: - bcachefs doesn't even report a maximum number of inodes (IIRC); inodes are small and variable size (most fields are varints, typical inode size is 50-100 bytes). - and the kernel has a sysctl to limit the maximum number of open files, and it's got a sane default; this is what's supposed to save us from pinned inodes eating up all ram), but systemd conveniently overwrites it to some absurd value... I'd prefer to see this fixed properly, rather than just "fixing" the test; userspace being able to pin all kernel memory this easily is a real bug. We could put a second hard cap on the maximum number of open files, and base that on a percentage of total memory; a VFS inode is somewhere in the ballpack of a kilobyte, so easy enough to calculate. And we could make that percentage itself a sysctl, for the people who are really crazy...
