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

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Date:   Thu, 27 Jan 2022 08:47:06 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Christoph Hellwig" <hch@...radead.org>
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, 24 Jan 2022, Christoph Hellwig wrote:
> 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>

Thanks.
> 
> > Fixes-no-auto-backport: 3fb5c298b04e ("swap: allow swap readahead to be merged")
> 
> Is this really a thing?
Maybe it should be.....
As I'm sure you guessed, I think it is valuable to record this
connection between commits, but I don't like it hasty automatic
backporting of patches where the (unknown) risk might exceed the (known)
value.  This is how I choose to state my displeasure.

Thanks,
NeilBrown