On Wed, Feb 21, 2024 at 07:07:37PM -0500, Kent Overstreet wrote: > 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. > I don't know bcache that well, but I didn't see anything obviously putting links in the bdev dir when I last looked. It did look like it created some links between its own sysfs dirs, but maybe I misinterpreted the code. I don't think that any other fs with its own /sys/fs/ presence (i.e., XFS, ext4, btrfs) does this sort of thing, so I'm a little skeptical of the idea that userspace currently needs it (not necessarily that it couldn't use it for the better in the future). > 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_ > Ok, but that sounds contradictory to what the block layer folks want. I dunno, it seems to me that the notions of better general coordination between /sys/block and /sys/fs and that of bcachefs' current behavior being wrong are not mutually exclusive things. But I'll just leave it alone until there's some more clarity here.. Brian
