linux-kernel - Re: [PATCH 02/23] MM: extend block-plugging to cover all swap reads with read-ahead

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Message-ID: <Ye5UzEzvN8WWMNBn@infradead.org>
Date:   Sun, 23 Jan 2022 23:27:08 -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>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/23] MM: extend block-plugging to cover all swap reads
 with read-ahead

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@....de>

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

Is this really a thing?