On Mon, Jan 11, 2016 at 7:24 AM, Hannes Reinecke <hare@xxxxxxx> wrote: > On 01/09/2016 08:54 AM, Al Viro wrote: >> >> On Mon, Jan 04, 2016 at 10:20:05AM -0800, Dan Williams wrote: >>> >>> Historically we have waited for filesystem specific heuristics to >>> attempt to guess when a block device is gone. Sometimes this works, but >>> in other cases the system can hang waiting for the fs to trigger its >>> shutdown protocol. >>> >>> The initial motivation for this investigation was to prevent DAX >>> mappings (direct mmap access to persistent memory) from leaking past the >>> lifetime of the hosting block device. However, Dave points out that >>> these shutdown operations are needed in other scenarios. Quoting Dave: >>> >>> For example, if we detect a free space corruption during allocation, >>> it is not safe to trust *any active mapping* because we can't trust >>> that we having handed out the same block to multiple owners. Hence >>> on such a filesystem shutdown, we have to prevent any new DAX >>> mapping from occurring and invalidate all existing mappings as we >>> cannot allow userspace to modify any data or metadata until we've >>> resolved the corruption situation. >>> >>> The current block device shutdown sequence of del_gendisk + >>> blk_cleanup_queue is problematic. We want to tell the fs after >>> blk_cleanup_queue that there is no possibility of recovery, but by that >>> time we have deleted partitions and lost the ability to find all the >>> super-blocks on a block device. >>> >>> Introduce del_gendisk_queue to trigger ->quiesce() and ->bdi_gone() >>> notifications to all the filesystems hosted on the disk. Where >>> ->quiesce() are 'shutdown' operations while the bdev may still be alive, >>> and ->bdi_gone() is a set of actions to take after the backing device >>> is known to be permanently dead. >> >> >> Would you mind explaining what the hell is _the_ backing device >> of a filesystem? What does that translate into in case of e.g. btrfs >> spanning several disks? Or ext4 with journal on a different device, for >> that matter? >> >> If anything, I would argue that filesystem is out of place here - >> general situation is "IO on X may require IO on device Y and X needs to do >> something when Y goes away". Consider e.g. /dev/loop backed by a device >> that went away. Or by a file on fs that has run down the curtain and >> joined >> the bleedin choir invisible. With another fs partially hosted by that >> loopback device. Or by RAID0 containing said device. >> >> You are given Y and attempt to locate the affected X. _Then_ >> you assume that X is a filesystem and has "something to be done" >> independent >> from the role Y played for it, so you can pick that action from superblock >> method. >> >> IMO you are placing the burden in the wrong place. _Recepient_ >> knows what it depends upon and what should be done for each source of >> trouble. So make it recepient's responsibility to request notifications. >> At which point the superblock method goes away, along with the requirement >> to handle all sources of trouble the same way, etc. >> >> What's more, things like RAID5 (also interested in knowing when >> a component has been ripped out) might or might not decide to propagate >> the event further - after all, that's exactly the point of redundancy. >> >> I'd look into something along the lines of notifier chain per >> gendisk, with potential victims registering a callback when they decide >> that from now on such and such device might screw them over... > > > Fully support this. I was planning on something similar to transport device > changes (resizing, topology change etc). > > And it might even be an idea to convert the block device events to a > notifier chain, too. > > Dan, can you keep me in the loop here? Yes, will do. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
