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

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Date:   Thu, 28 Jan 2021 00:28:37 -0800
From:   Chris Goldsworthy <cgoldswo@...eaurora.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Minchan Kim <minchan@...nel.org>, viro@...iv.linux.org.uk,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Laura Abbott <lauraa@...eaurora.org>
Subject: Re: [PATCH v4] fs/buffer.c: Revoke LRU when trying to drop buffers

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

Thanks,

Chris.

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