linux-kernel - Re: [RFC PATCH] mm/readahead: readahead aggressively if read drops in willneed range

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Date: Sun, 28 Jan 2024 22:02:49 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Ming Lei <ming.lei@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Mike Snitzer <snitzer@...nel.org>,
	Don Dutile <ddutile@...hat.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH] mm/readahead: readahead aggressively if read drops
 in willneed range

On Sun, Jan 28, 2024 at 10:25:22PM +0800, Ming Lei wrote:
> Since commit 6d2be915e589 ("mm/readahead.c: fix readahead failure for
> memoryless NUMA nodes and limit readahead max_pages"), ADV_WILLNEED
> only tries to readahead 512 pages, and the remained part in the advised
> range fallback on normal readahead.

Does the MAINTAINERS file mean nothing any more?

> If bdi->ra_pages is set as small, readahead will perform not efficient
> enough. Increasing read ahead may not be an option since workload may
> have mixed random and sequential I/O.

I thik there needs to be a lot more explanation than this about what's
going on before we jump to "And therefore this patch is the right
answer".

> @@ -972,6 +974,7 @@ struct file_ra_state {
>  	unsigned int ra_pages;
>  	unsigned int mmap_miss;
>  	loff_t prev_pos;
> +	struct maple_tree *need_mt;

No.  Embed the struct maple tree.  Don't allocate it.  What made you
think this was the right approach?