Re: bcache-register hang after reboot

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On Fri, Nov 12, 2021 at 01:45:06PM +0800, Coly Li wrote:
> On 11/11/21 4:13 AM, Kent Overstreet wrote:
> > On Wed, Nov 10, 2021 at 11:14:41AM +0530, Nikhil Kshirsagar wrote:
> > > Hello,
> > > 
> > > After a reboot of an Ubuntu server running 4.15.0-143-generic kernel,
> > > the storage devices using bcache do not come back up and the following
> > > stack traces are seen in kern.log. Please could someone help me
> > > understand if this is due to a full bcache journal? Is there any
> > > workaround, or fix?
> > 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.
> 
> This happens eventually. I had a patch to reserve the last 2 blocks in the
> last journal bucket, and only to use them during boot time. It seems to be
> time to refine the patch.

bcachefs has that modification as well - the bcache journal code is so minimal
in comparison.

> (BTW, looking forward to see bcachefs in mainline kernel).

Thanks! Now that snapshots are done, I'm going to be focusing on closing bugs
for awhile, and then hopefully finally upstreaming it soon.



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