On Thu, 19 Oct 2023 at 09:17, Kent Overstreet <[email protected]> wrote: > > On Tue, Oct 10, 2023 at 11:20:59PM +0800, Coly Li wrote: > > Forward it to [email protected] > > <mailto:[email protected]> > > > > > 2023年10月10日 07:43，Daniel J Blueman <[email protected]> 写道： > > > > > > Firstly, bcachefs introduces a new era of in-tree filesystems with > > > some monumental features (sorry, ZFS); hats off to Kent for landing > > > this! > > > > > > My testing finds it is in great shape; far better than BTRFS was when > > > it landed. Testing on linux next-20231005 with additional debug checks > > > atop the Ubuntu 23.04 kernel generic config [1], I was able to provoke > > > a btree trans path overflow cornercase [2]. > > > > > > The minimal reproducer is: > > > # modprobe brd rd_nr=2 rd_size=1048576 > > > # bcachefs format --metadata_replicas=2 --label=tier1.1 /dev/ram0 > > > --label=tier1.2 /dev/ram1 > > > # mount -t bcachefs /dev/ram0:/dev/ram1 /mnt > > > # dd if=/dev/zero of=/mnt/test bs=128M > > > > > > The issue doesn't reproduce with metadata_replicas=1 or a single block > > > device. > > > > > > At debug entry, I couldn't determine why BTREE_ITER_MAX must be 64 > > > rather than 32 when CONFIG_LOCKDEP is set, however the panic doesn't > > > occur without CONFIG_LOCKDEP, so it appears related; keeping it at > > > value 32 with CONFIG_LOCKDEP doesn't prevent the panic also. > > Lockup can only track up to 48 held locks but each btree path can hold a > lock - hence we need to limit the number of max paths for lockdep to > function. > > In the main branch there's a patch that ups the lockdep limit to 63 and > drops the BTREE_ITER_MAX=32. > > The underlying issue is a bit more serious though; it is becoming > increasingly impractical to count the number of paths we're going to > need ahead of time so that we can run inside of a fixed limit. There's a > number of extra btrees for allocation purposes that we only hit > sometimes, and we really can't predict ahead of time which ones will > need to be updated. > > Long term, we're probably going to need to figure out a way to > dynamically allocate btree paths and get rid of the fixed limit.
Fantastic info Kent; this explains it; I'll revisit lockdep testing on bcachefs when there are related changes later. Thanks, Dan -- Daniel J Blueman
