linux-kernel - Re: [PATCH] aio: Add command to wait completion of all requests

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Date:   Tue, 13 Jun 2017 19:26:45 -0400
From:   Benjamin LaHaise <bcrl@...ck.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     avagin@...nvz.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk, gorcunov@...nvz.org,
        akpm@...ux-foundation.org, xemul@...tuozzo.com
Subject: Re: [PATCH] aio: Add command to wait completion of all requests

On Tue, Jun 13, 2017 at 07:17:43PM +0300, Kirill Tkhai wrote:
> On 13.06.2017 18:26, Benjamin LaHaise wrote:
> > On Tue, Jun 13, 2017 at 06:11:03PM +0300, Kirill Tkhai wrote:
> > ...
> >> The functionality, I did, grew from real need and experience. We try to
> >> avoid kernel modification, where it's possible, but the in-flight aio
> >> requests is not a case suitable for that.
> > 
> > What you've done only works for *your* use-case, but not in general.  Like
> > in other subsystems, you need to provide hooks on a per file descriptor
> > basis for quiescing different kinds of file descriptors.
> 
> Which hooks do you suggest? It's possible there is no a file descriptor open after
> a request is submitted. Do you suggest an interface to reopen a struct file?

That's what you have to contend with.  AIO keeps the struct file pinned
for the life of the request.  This is one of the complex issues you need
to address.

> > Your current
> > patch set completely ignores things like usb gadget.  You need to create
> > infrastructure for restarting i/os after your checkpointing occurs, which
> > you haven't put any thought into in this patchset.  If you want to discuss
> > how to do that, fine, but the approach in this patchset simply does not
> > work in general.  What happens when an aio doesn't complete or takes hours
> > to complete?
> 
> Here is wait_event_interruptible(), but it's possible to convert it
> in wait_event_interruptible_hrtimeout() like it's made in read_events().
> It's not a deadly issue of patch. The function read_events() simply
> waits for timeout, can't we do the same?

Nope.  An aio may not complete in a timely fashion, in which case your
wait for all aios to complete will simply wait forever.  I take it this is
not the desired behaviour of checkpointing.

> Could you please describe how will cancelling aio requests will help to wait
> till their completion? Is there is guarantee, they will be easily queued back?
> I suppose, no, because there are may be a memory limit or some low level
> drivers limitations, dependent on internal conditions.

This is the problem you're facing.  Quiescing aios is complicated!

> Also, it's not seems good to overload aio with the functionality of obtaining
> closed file descriptors of submitted requests.
> 
> Do you mean this way, or I misunderstood you? Could you please to concretize your
> idea?

Yes, this is what I'm talking about.

> In my vision cancelling requests does not allow to implement the need I described.
> If we can't queue request back, it breaks snapshotting and user application in
> general.

This is what you have to figure out.  This is why your patch is incomplete
and cannot be accepted.  You can't punt the complexity your feature
requires onto other maintainers -- your implementation has to be reasonably
complete at time of patch inclusion.  Can you see now why your patch can't
be included as-is?  The assumption you made that aios complete in a timely
fashion is incorrect.  Everything else stems from that.

		-ben

> Thanks,
> Kirill
> 

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