Re: [PATCH v4] fs/buffer.c: Revoke LRU when trying to drop buffers

[Minchan Kim]

On Thu, Jan 28, 2021 at 12:28:37AM -0800, Chris Goldsworthy wrote:
> On 2021-01-26 18:59, Matthew Wilcox wrote:
> > On Tue, Jan 26, 2021 at 02:59:17PM -0800, Minchan Kim wrote:
> > > The release buffer_head in LRU is great improvement for migration
> > > point of view.
> > > 
> > > A question:
> 
> Hey guys,
> 
> > > 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.