On Wed, 13 Jan 2021 13:17:30 -0800 Chris Goldsworthy <cgoldswo@xxxxxxxxxxxxxx> wrote: > From: Laura Abbott <lauraa@xxxxxxxxxxxxxx> > > When a buffer is added to the LRU list, a reference is taken which is > not dropped until the buffer is evicted from the LRU list. This is the > correct behavior, however this LRU reference will prevent the buffer > from being dropped. This means that the buffer can't actually be dropped > until it is selected for eviction. There's no bound on the time spent > on the LRU list, which means that the buffer may be undroppable for > very long periods of time. Given that migration involves dropping > buffers, the associated page is now unmigratible for long periods of > time as well. CMA relies on being able to migrate a specific range > of pages, so these types of failures make CMA significantly > less reliable, especially under high filesystem usage. > > Rather than waiting for the LRU algorithm to eventually kick out > the buffer, explicitly remove the buffer from the LRU list when trying > to drop it. There is still the possibility that the buffer > could be added back on the list, but that indicates the buffer is > still in use and would probably have other 'in use' indicates to > prevent dropping. > > Note: a bug reported by "kernel test robot" lead to a switch from > using xas_for_each() to xa_for_each(). (hm, why isn't drop_buffers() static to fs/buffer.c??) It looks like patch this turns drop_buffers() into a very expensive operation. And that expensive operation occurs under the address_space-wide private_lock, which is more ouch. How carefully has this been tested for performance? In pathological circumstances (which are always someone's common case :() Just thinking out loud... If a buffer_head* is sitting in one or more of the LRUs, what is stopping us from stripping it from the page anyway? Then try_to_free_buffers() can mark the bh as buffer_dead(), declare success and leave the bh sitting in the LRU, with the LRU as the only reference to that buffer. Teach lookup_bh_lru() to skip over buffer_dead() buffers and our now-dead buffer will eventually reach the tail of the lru and get freed for real.
