On Sun, Apr 28, 2024 at 06:12:32AM +0100, Al Viro wrote: > We have several bool members in struct block_device. > It would be nice to pack that stuff, preferably without > blowing the cacheline boundaries. > > That had been suggested a while ago, and initial > implementation by Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> ran into > objections re atomicity, along with the obvious suggestion to > use unsigned long and test_bit/set_bit/clear_bit for access. > Unfortunately, that *does* blow the cacheline boundaries. > > However, it's not hard to do that without bitops; > we have an 8-bit assign-once partition number nearby, and > folding it into a 32-bit member leaves us up to 24 bits for > flags. Using cmpxchg for setting/clearing flags is not > hard, and 32bit cmpxchg is supported everywhere. To me this looks pretty complicated and arcane. How about we just reclaim a little space in the block device and just keep bd_partno and add an unsigned long base flags? E.g. bd_meta_info is only used in slow path early boot and sysfs code and just set for a few partitions. Just turn it into a hash/xrray/whatever indexed by bd_dev and we've already reclaimed enough space.
