On Fri, Feb 07, 2025 at 05:54:46AM +0100, Christoph Hellwig wrote: > On Thu, Feb 06, 2025 at 08:31:23PM -0800, Darrick J. Wong wrote: > > On Thu, Feb 06, 2025 at 07:44:50AM +0100, Christoph Hellwig wrote: > > > While the zoned on-disk format supports reflinks, the GC code currently > > > always unshares reflinks when moving blocks to new zones, thus making the > > > feature unusuable. Disable reflinks until the GC code is refcount aware. > > > > It might be worth mentioning that I've been working on a refcount-aware > > free space defragmenter that could be used for this kind of thing, > > albeit with the usual problems of userspace can't really stop the > > kernel from filling its brains and starving our defragc process. > > > > It would be interesting to load up that adversarial thread timing > > sched_ext thing that I hear was talked about at fosdem. > > Not sure waht sched_ext thing and how it's relevant. https://lwn.net/Articles/1007689/ IOWs letting the wrong threads run ahead, to see if the author got the locking right. > It's just that refcount awareness is a bit of work and not really > required for the initial use cases. It might also have fun implications > for the metabtree reservations, but otherwise I don't think it's rocket > science. Even more so with a pre-existing implementation to steal ideas > from. > > Talking about stealing ideas - the in-kernel GC flow should mostly work > for a regular file system as well. I don't think actually sharing much > code is going to be useful because the block reservations work so > differently, but I suspect doing it in-kernel using the same "check if > the mapping change at I/O completion time" scheme use in GC would work > very well for a freespace defragmenter on a regular file system. And > doing it in the kernel will be more efficient at operation time and > probably also be a lot less code than doing it with a whole bunch of > userspace to kernel roundtrips. Admittedly there are some fun things that you can do in the kernel that you can't do from userspace, like create a secret unlinked file that you can map freespace into, and turn EFIs into a bmapi operation, and reflink file blocks into without having to deal with eof blocks, and turning on alwayscow for files you're actively trying to move around. But that's a fun thing to think about now that I have far fewer patches to juggle in my head, for which I am grateful. :) --D
