linux-kernel - Re: scsi-mq V2

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Date:	Thu, 10 Jul 2014 22:06:52 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	Benjamin LaHaise <bcrl@...ck.org>,
	"Elliott\, Robert \(Server Storage\)" <Elliott@...com>,
	Christoph Hellwig <hch@...radead.org>,
	"dgilbert@...erlog.com" <dgilbert@...erlog.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Bart Van Assche <bvanassche@...ionio.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: scsi-mq V2

On 2014-07-10 22:05, Jeff Moyer wrote:
> Jens Axboe <axboe@...nel.dk> writes:
>
>> On 2014-07-10 17:11, Jeff Moyer wrote:
>>> Benjamin LaHaise <bcrl@...ck.org> writes:
>>>
>>>>>
>>>>> [  186.339064] ioctx_alloc: nr_events=-2 aio_max_nr=65536
>>>>> [  186.339065] ioctx_alloc: nr_events=-2 aio_max_nr=65536
>>>>> [  186.339067] ioctx_alloc: nr_events=-2 aio_max_nr=65536
>>>>> [  186.339068] ioctx_alloc: nr_events=-2 aio_max_nr=65536
>>>>> [  186.339069] ioctx_alloc: nr_events=-2 aio_max_nr=65536
>>>>
>>>> Something is horribly wrong here.  There is no way that value for nr_events
>>>> should be passed in to ioctx_alloc().  This implies that userland is calling
>>>> io_setup() with an impossibly large value for nr_events.  Can you post the
>>>> actual diff for your fs/aio.c relative to linus' tree?
>>>>
>>>
>>> fio does exactly this!  it passes INT_MAX.
>>
>> That's correct, I had actually forgotten about this. It was a change
>> made a few years back, in correlation with the aio optimizations
>> posted then, basically telling aio to ignore that silly (and broken)
>> user ring.
>
> I still don't see how you accomplish that.  Making it bigger doesn't get
> rid of it.  ;-)

See the patches from back then - INT_MAX basically just meant the same 
as 0, but 0 could not be used because of the (silly) setup with the 
wrappers around the syscalls. So INT_MAX was overloaded to mean "no ring 
events, I don't care".

-- 
Jens Axboe

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