linux-kernel - Re: [PATCH 7/9] aio: add aio_kernel

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Message-ID: <CACVXFVPXTQyipPLrQML6ONwD3b4g4nHWc=OVjkUMZOiv5daevg@mail.gmail.com>
Date:	Thu, 24 Jul 2014 09:57:16 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Zach Brown <zab@...bo.net>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"open list:AIO" <linux-aio@...ck.org>,
	Kent Overstreet <kmo@...erainc.com>
Subject: Re: [PATCH 7/9] aio: add aio_kernel_() interface

On Thu, Jul 24, 2014 at 7:16 AM, Zach Brown <zab@...bo.net> wrote:
> On Thu, Jul 24, 2014 at 06:55:28AM +0800, Ming Lei wrote:
>> From: Dave Kleikamp <dave.kleikamp@...cle.com>
>>
>> This adds an interface that lets kernel callers submit aio iocbs without
>> going through the user space syscalls.  This lets kernel callers avoid
>> the management limits and overhead of the context.  It will also let us
>> integrate aio operations with other kernel apis that the user space
>> interface doesn't have access to.
>>
>> This patch is based on Dave's posts in below links:
>>
>>       https://lkml.org/lkml/2013/10/16/365
>>       https://groups.google.com/forum/#!topic/linux.kernel/l7mogGJZoKQ
>
> This was originally written a billion years ago when dinosaurs roamed
> the earth.  Also, notably, before Kent and Ben reworked a bunch of the

Not so far away, this patch is based on Dave's last version of V9, which
was posted in Oct, 2013, :-)

> aio core.  I'd want them to take a look at this patch to make sure that
> it doesn't rely on any assumptions that have changed.

Looks I missed to Cc Ken, :-(

>
>> +/* opcode values not exposed to user space */
>> +enum {
>> +     IOCB_CMD_READ_ITER = 0x10000,
>> +     IOCB_CMD_WRITE_ITER = 0x10001,
>> +};
>
> And I think the consensus was that this isn't good enough.  Find a way
> to encode the kernel caller ops without polluting the uiocb cmd name
> space.

That is easy, since the two cmd names are only for kernel AIO, whatever
should be OK, but looks I didn't see such comment.

>
> (I've now come to think that this entire approach of having loop use aio
> is misguided and that the way forward is to have dio consume what loop
> naturally produces -- bios, blk-mq rqs, whatever -- but I'm on to other

Yes, that is what these patches are doing, and actually AIO's
model is a good match to driver's interface. Lots of drivers
use the asynchronous model(submit, complete, ...).

> things these days.)

At least, loop can improve its throughput much by kernel AIO
without big changes to fs/direct-io(attribute much to ITER_BVEC),
and vhost-scsi should benefit from it too.

Thanks,
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