On Thu, Jan 28, 2021 at 12:28:37AM -0800, Chris Goldsworthy wrote:

On 2021-01-26 18:59, Matthew Wilcox wrote:

quoted

On Tue, Jan 26, 2021 at 02:59:17PM -0800, Minchan Kim wrote:

quoted

The release buffer_head in LRU is great improvement for migration
point of view.

A question:

Hey guys,

quoted

quoted

Can't we invalidate(e.g., invalidate_bh_lrus) bh_lru in migrate_prep
or
elsewhere when migration found the failure and is about to retry?

Migration has done such a way for other per-cpu stuffs for a long
time,
which would be more consistent with others and might be faster
sometimes
with reducing IPI calls for page.

Should lru_add_drain_all() also handle draining the buffer lru for all
callers?  A quick survey ...

invalidate_bdev() already calls invalidate_bh_lrus()
compact_nodes() would probably benefit from the BH LRU being invalidated
POSIX_FADV_DONTNEED would benefit if the underlying filesystem uses BHs
check_and_migrate_cma_pages() would benefit
khugepaged_do_scan() doesn't need it today
scan_get_next_rmap_item() looks like it only works on anon pages (?) so
	doesn't need it
mem_cgroup_force_empty() probably needs it
mem_cgroup_move_charge() ditto
memfd_wait_for_pins() doesn't need it
shake_page() might benefit
offline_pages() would benefit
alloc_contig_range() would benefit

Seems like most would benefit and a few won't care.  I think I'd lean
towards having lru_add_drain_all() call invalidate_bh_lrus(), just to
simplify things.

Doing this sounds like a good idea.  We would still need a call to
invalidate_bh_lrus() inside of drop_buffers() in the event that we find
busy buffers, since they can be re-added back into the BH LRU - I believe
it isn't until this point that a BH can't be added back into the BH LRU,
when we acquire the private_lock for the mapping:

https://elixir.bootlin.com/linux/v5.10.10/source/fs/buffer.c#L3240

I am not sure it's good deal considering IPI overhead per page release
at worst case.

A idea is to disable bh_lrus in migrate_prep and enable it when
migration is done(need to introduce "migrate_done".
It's similar approach with marking pageblock MIGRATE_ISOLATE to
disable pcp during the migration.