linux-kernel - Re: [PATCH 12/13] MM: use AIO/DIO for reads from SWP_FS

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Date:   Wed, 17 Nov 2021 08:46:14 +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>, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCH 12/13] MM: use AIO/DIO for reads from SWP_FS_OPS swap-space

On Tue, 16 Nov 2021, Christoph Hellwig wrote:
> On Tue, Nov 16, 2021 at 01:44:04PM +1100, NeilBrown wrote:
> > When pages a read from SWP_FS_OPS swap-space, the reads are submitted as
> > separate reads for each page.  This is generally less efficient than
> > larger reads.
> > 
> > We can use the block-plugging infrastructure to delay submitting the
> > read request until multiple contigious pages have been collected.  This
> > requires using ->direct_IO to submit the read (as ->readpages isn't
> > suitable for swap).
> 
> Abusing the block code here seems little ugly.  Also this won't
> compile if CONFIG_BLOCK is not set, will it?

There is nothing really block-layer-specific about the plugging
interfaces.  I think it would be quite reasonable to move them to lib/
But you are correct that currently without CONFIG_BLOCK the code will
compile but not work.

> 
> What is the problem with just batching up manually?

That would require a bigger change to common code, which would only
benefit one user.  The plugging mechanism works well for batching
requests to a block device.  Why not use it for non-block-devices too?

Thanks,
NeilBrown


> 
> > +	/* nofs needs as ->direct_IO may take the same mutex it takes for write */
> 
> Overly long line.
> 
>