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

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Date:   Wed, 27 Jan 2021 02:59:22 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Chris Goldsworthy <cgoldswo@...eaurora.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 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: 
> 
> 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.