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Message-ID: <20140814180751.GB429@lenny.home.zabbo.net>
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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