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Date:   Sun, 23 Jan 2022 23:27:08 -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>,
        Christoph Hellwig <hch@infradead.org>,
        David Howells <dhowells@redhat.com>, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 02/23] MM: extend block-plugging to cover all swap reads
 with read-ahead
Message-ID: <Ye5UzEzvN8WWMNBn@infradead.org>
References: <164299573337.26253.7538614611220034049.stgit@noble.brown>
 <164299611274.26253.13900771841681128440.stgit@noble.brown>
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On Mon, Jan 24, 2022 at 02:48:32PM +1100, NeilBrown wrote:
> Code that does swap read-ahead uses blk_start_plug() and
> blk_finish_plug() to allow lower levels to combine multiple read-ahead
> pages into a single request, but calls blk_finish_plug() *before*
> submitting the original (non-ahead) read request.
> This missed an opportunity to combine read requests.
> 
> This patch moves the blk_finish_plug to *after* all the reads.
> This will likely combine the primary read with some of the "ahead"
> reads, and that may slightly increase the latency of that read, but it
> should more than make up for this by making more efficient use of the
> storage path.
> 
> The patch mostly makes the code look more consistent.  Performance
> change is unlikely to be noticeable.

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

> Fixes-no-auto-backport: 3fb5c298b04e ("swap: allow swap readahead to be merged")

Is this really a thing?