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

[Chris Goldsworthy <cgoldswo@codeaurora.org>]

On 2021-01-28 09:08, Minchan Kim wrote:
> 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.

I'll try creating that mechanism then for the BH LRU, and will come
back with a patch that does the invalidate in lru_add_drain_all().

Thanks Matthew and Minchan for the feedback!

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