Date:   Tue, 16 Nov 2021 00:31:59 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Chuck Lever <chuck.lever@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-block@vger.kernel.org
Subject: Re: [PATCH 12/13] MM: use AIO/DIO for reads from SWP_FS_OPS
 swap-space
Message-ID: <YZNsf5yvfb8+SiqB@infradead.org>
References: <163702956672.25805.16457749992977493579.stgit@noble.brown>
 <163703064458.25805.6777856691611196478.stgit@noble.brown>
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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.