linux-kernel - Re: [PATCH 12/13] MM: use AIO/DIO for reads from SWP_FS

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Message-ID: <YZNsf5yvfb8+SiqB@infradead.org>
Date:   Tue, 16 Nov 2021 00:31:59 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     NeilBrown <neilb@...e.de>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCH 12/13] MM: use AIO/DIO for reads from SWP_FS_OPS
 swap-space

On Tue, Nov 16, 2021 at 01:44:04PM +1100, NeilBrown wrote:
> When pages a read from SWP_FS_OPS swap-space, the reads are submitted as
> separate reads for each page.  This is generally less efficient than
> larger reads.
> 
> We can use the block-plugging infrastructure to delay submitting the
> read request until multiple contigious pages have been collected.  This
> requires using ->direct_IO to submit the read (as ->readpages isn't
> suitable for swap).

Abusing the block code here seems little ugly.  Also this won't
compile if CONFIG_BLOCK is not set, will it?

What is the problem with just batching up manually?

> +	/* nofs needs as ->direct_IO may take the same mutex it takes for write */

Overly long line.