On Thu, Nov 11, 2021 at 05:54:18PM -0300, Mauricio Oliveira wrote: > Hi Kent, > > On Wed, Nov 10, 2021 at 5:13 PM Kent Overstreet > <kent.overstreet@xxxxxxxxx> wrote: > > Your journal is completely full, so persisting the new btree root while doing > > journal replay is hanging. > > > > There isn't a _good_ solution for this journal deadlock in bcache (it's fixed in > > bcachefs), but there is a hack: > > > > edit drivers/md/bcache/btree.c line 2493 > > > > delete the call to bch_journal_meta(), and build a new kernel. Once you've > > gotten it to register, do a clean shutdown and then go back to a stock kernel. > > > > Running the kernel with that call deleted won't be safe if you crash, but it'll > > get you going again. > > Thanks for the clarification and suggestions. > > Would it be OK to implement that workaround if requested by a sysadmin ? > (say, to ack the data safety / crash risk) > > Right now the issue is known, reproduces with v5.15, has no good solution, > remains after reboot, prints hung task warnings continuously, and prevents > using the device at all; and this workaround requires kernel dev/build skills. > > Since its effects seem bad enough, it would seem fair enough to provide a > way out even if it's not a _good_ one. > > Say, we could try and detect the journal full during journal replay, and handle > it by failing the device registration. This would unblock the tasks, and provide > a more intuitive error message. (maybe leading to the next paragraph.) > > We could also add a sysfs tunable to skip the call to bch_journal_meta(), > and allow the registration to proceed, but fail it unconditionally in the end > so the device isn't used with data safety / crash risk > (or force an automatic unregister + register again w/ bch_journal_meta(), > and disable the sysfs tunable). > > This would help with the full journal, and allow a sysadmin to perform the > workaround without kernel rebuild and reboots. I think the best solution might be to change bch_btree_set_root() to check if we're in journal replay, and if we are, make the call to bch_journal_meta() nonblocking - pass it NULL instead of a closure.
