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

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Message-ID: <20190718235616.GM29731@kvack.org>
Date:   Thu, 18 Jul 2019 19:56:16 -0400
From:   Benjamin LaHaise <ben@...munityfibre.ca>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>, 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, 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.
> 
> In this particular case I have a sysfs file that is accessing hardware
> and hence will block for a while and using this patch I can io_submit()
> a write and handle the completion of this in my normal event loop.
> 
> 
> Each individual io operation will be just as synchronous as the current
> iter-based mechanism - for the drivers that implement that.

Just adding the fops is not enough.  I have patches floating around at
Solace that add thread based fallbacks for files that don't have an aio
read / write implementation, but I'm not working on that code any more.
The thread based methods were quite useful in applications that had a need
for using other kernel infrastructure in their main event loops.

		-ben

> Regards,
> Bjorn
> 

-- 
"Thought is the essence of where you are now."