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 11:32:02 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Hugh Dickins" <hughd@...gle.com>
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, Hugh Dickins wrote:
> 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).
> 
....
> 
> 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.

Thanks - I had missed that.  The code is correct, but looks wrong (to
me).
I've dropped the patch, but added a comment when I add
"swap_read_unplug()" to explain while plugging isn't needed for that
final read_swap_cache_async().

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

:-)

Thanks,
NeilBrown