linux-kernel - Re: [PATCH] aio: Support read/write with non-iter file-ops

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Message-ID: <20190719000702.GW17978@ZenIV.linux.org.uk>
Date:   Fri, 19 Jul 2019 01:07:02 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Benjamin LaHaise <bcrl@...ck.org>, linux-fsdevel@...r.kernel.org,
        linux-aio@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] aio: Support read/write with non-iter file-ops

On Thu, Jul 18, 2019 at 04:43:52PM -0700, Bjorn Andersson wrote:
> On Thu 18 Jul 16:17 PDT 2019, Al Viro wrote:
> 
> > On Thu, Jul 18, 2019 at 04:10:54PM -0700, Bjorn Andersson wrote:
> > > Implement a wrapper for aio_read()/write() to allow async IO on files
> > > not implementing the iter version of read/write, such as sysfs. This
> > > mimics how readv/writev uses non-iter ops in do_loop_readv_writev().
> > 
> > IDGI.  How would that IO manage to be async?  And what's the point
> > using aio in such situations in the first place?
> 
> The point is that an application using aio to submit io operations on a
> set of files,

... for no reason whatsoever, I take it?

> can use the same mechanism to read/write files that
> happens to be implemented by driver only implementing read/write (not
> read_iter/write_iter) in the registered file_operations struct, such as
> kernfs.

... except that it still has to support the kernels that don't have
your patch, so the fallback in userland is *not* going away.