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

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Date:   Wed, 26 Jan 2022 15:09:43 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     NeilBrown <neilb@...e.de>
cc:     Christoph Hellwig <hch@...radead.org>,
        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>,
        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 Thu, 27 Jan 2022, NeilBrown wrote:
> 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.

No, you're misunderstanding there.  All the necessary reads are issued
within the loop, between the plug and unplug: it does not skip over
the target page in the loop, but issues its read along with the rest.

But it has not kept any of those pages locked, nor even kept any
refcounts raised: so at the end has to look up the target page again
with the final read_swap_cache_async() (which also copes with the
highly unlikely case that the page got swapped out again meanwhile).

> > > 
> > > 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.

I don't suppose your patch does any actual harm (beyond propagating a
misunderstanding), but it's certainly not a fix, and I think should
simply be dropped from the series.

(But please don't expect any comment from me on the rest:
SWP_FS_OPS has always been beyond my understanding.)

Hugh