Re: kernel oops on bcachefs umount, 6.7 kernel

Kent Overstreet
2024-02-22T00:07:42Z

On Wed, Feb 21, 2024 at 07:39:49AM -0500, Brian Foster wrote:
> On Fri, Feb 16, 2024 at 10:57:24AM -0500, Kent Overstreet wrote:
> > On Fri, Feb 16, 2024 at 07:40:34AM -0500, Brian Foster wrote:
> > > On Fri, Feb 16, 2024 at 09:00:17AM +0100, Christoph Hellwig wrote:
> > > > On Thu, Feb 15, 2024 at 07:24:23PM -0500, Kent Overstreet wrote:
> > > > > > It looks like the warning could be avoided in bcachefs by checking for
> > > > > > whether the parent dir/node still exists at cleanup time, but I'm not
> > > > > > familiar enough with kobj management to say whether that's the
> > > > > > right/best solution. It also looks a little odd to me to see a
> > > > > > /sys/block/<dev>/bcachefs dir when I've not seen any other fs or driver
> > > > > > do such a thing in the block sysfs dir(s).
> > > > > >
> > > > > > Any thoughts on this from the block subsystem folks? Is it reasonable to
> > > > > > leave this link around and just fix the removal check, or is another
> > > > > > behavior preferred? Thanks.
> > > >
> > > > This is the general problem with random cross-subsystem sysfs reference,
> > > > and why they are best avoided. The block layer tears down all the sysfs
> > > > objects at del_gendisk time as no one should start using the sysfs files
> > > > at that point, but a mounted file system or other opener will of course
> > > > keep the bdev itself alive.
> > > >
> > >
> > > Yeah, makes sense. The fact that the dir goes away despite having the
> > > bdev open is partly what made this seem a little odd to me.
> > >
> > > > I'm not sure why bcachefs is doing this, but no one really should be
> > > > using the block layer sysfs structures and pointers except for the block
> > > > layer itself.
> > > >
> > >
> > > From Kent's comments it sounds like it was just some loose carryover
> > > from old bcache stuff. I had poked around a bit for anything similar and
> > > it looked to me that current bcache doesn't do this either, but I could
> > > have missed something.
> > >
> > > > > so there's an existing bd_holder mechanism that e.g. device mapper uses
> > > > > for links between block devices. I think the "this block device is going
> > > > > away" code knows how to clean those up.
> > > > >
> > > > > We're not using that mechanism - and perhaps we should have been, I'd
> > > > > need a time machine to ask myself why I did it that way 15 years back.
> > > >
> > > > Well, at least Tejun had a very strong opinion that no one should be
> > > > abusing sysfs symlinks for linking up subsystems at all, see commit
> > > > 49731baa41df404c2c3f44555869ab387363af43, which is also why this code
> > > > is marked deprecated and we've not added additional users.
> > > >
> > >
> > > Thanks. I'll send a patch to remove this once I'm back from vacation.
> >
> > No, we can't remove it - userspace needs to know this topology. When
> > we've got one sysfs node with a direct relationship to another sysfs
> > node, that needs to be reflected in syfs.
> >
>
> Do you mean that some related userspace tool relies on this to function,
> or generally disagreeing with the statement(s) above around links from
> /sys/block/<dev>/?

I would have to do some digging to give a definitive answer to that
question.

bcache definitely needed those links (IIRC in udev rules?) - bcachefs
may not, but we haven't even started on integrating bcachefs with all
the userspace disk management tooling out there.

And this is stuff that userspace does in general need; if we're getting
by without it for other filesystems right now it's probably by doing
something horrid like parsing /proc/mounts; if we could get filesystems
using the same bd_holder stuff that other block layer stuff uses, that
would be _amazing_