linux-kernel - Re: [PATCH v1 1/9] aio: add aio_kernel

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Date:	Thu, 14 Aug 2014 11:07:51 -0700
From:	Zach Brown <zab@...bo.net>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Christoph Hellwig <hch@...radead.org>,
	Kent Overstreet <kmo@...erainc.com>, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, Dave Chinner <david@...morbit.com>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v1 1/9] aio: add aio_kernel_() interface

On Thu, Aug 14, 2014 at 11:50:32PM +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

(And some other werido's posts, almost 5 entire earth years ago:
 http://permalink.gmane.org/gmane.linux.file-systems/36246)

> +struct kiocb *aio_kernel_alloc(gfp_t gfp, unsigned extra)
> +{
> +	return kzalloc(sizeof(struct kiocb) + extra, gfp);

Is kzalloc really necessary?  It's insane, but in the past we've had
people whine about the cycle costs of zeroing fields that are to be
initialized:

	commit 23aee091d804efa8cc732a31c1ae5d625e1ec886
	Author: Jeff Moyer <jmoyer@...hat.com>
	Date:   Tue Dec 15 16:47:49 2009 -0800

	    dio: don't zero out the pages array inside struct dio

Maybe add a guard value to the ctx and have submission freak out of it's
called without being initialized?  If callers really want to zero they
can pass in __GFP_ZERO.

The extra allocation at the end that's freed is nice, but the callers
having a clumsy manual cast to access it isn't nice at all.  Can you add
a little helper to get a pointer to the extra allocation?    That'd let
the aio bits allocation the iocbs however the like (slab, per-cpu,
whatever) and have extra allocations separate if that ends up making
sense.

> +	iocb->ki_ctx = (void *)-1;

The magic -1 is gross.  Use a constant?  (bonus points for having it use
ERR_PTR() :))

> +	/*
> +	 * use same policy with userspace aio, req may have been
> +	 * completed already, so release it by aio completion.
> +	 */
> +	if (ret != -EIOCBQUEUED)
> +		iocb->ki_obj.complete(iocb->ki_user_data, ret);

I wonder if this needs to handle the restarting error codes like
aio_complete() does.

	commit a0c42bac79731276c9b2f28d54f9e658fcf843a2
	Author: Jan Kara <jack@...e.cz>
	Date:   Wed Sep 22 13:05:03 2010 -0700

	    aio: do not return ERESTARTSYS as a result of AIO

I like how this has evolved to get rid of the magic key and commands..
just the ki_ctx and calling iter methods, nice stuff.

- z
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