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

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Date:   Thu, 18 Jul 2019 17:36:55 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Benjamin LaHaise <ben@...munityfibre.ca>
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 18 Jul 16:56 PDT 2019, Benjamin LaHaise wrote:

> 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.

My bad. Took another look and now I see the bigger picture of how this
is currently implemented and why just adding the fops would defeat the
purpose of the api.

Sorry for the noise.

> The thread based methods were quite useful in applications that had a need
> for using other kernel infrastructure in their main event loops.
> 

Yes indeed.

Regards,
Bjorn