linux-kernel - Re: [PATCH] aio: Fix return code of io

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Message-ID: <542EE753.20005@kernel.dk>
Date:	Fri, 03 Oct 2014 12:13:39 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Kent Overstreet <kmo@...erainc.com>, linux-kernel@...r.kernel.org
CC:	Benjamin LaHaise <bcrl@...ck.org>, Zach Brown <zab@...bo.net>,
	Jeff Moyer <jmoyer@...hat.com>, Slava Pestov <sp@...erainc.com>
Subject: Re: [PATCH] aio: Fix return code of io_submit() (RFC)

On 2014-10-03 12:08, Kent Overstreet wrote:
> io_submit() could return -EAGAIN on memory allocation failure when it should
> really have been returning -ENOMEM. This could confuse applications (i.e. fio)
> since -EAGAIN means "too many requests outstanding, wait until completions have
> been reaped" and if the application actually was tracking outstanding
> completions this wouldn't make a lot of sense.
>
> NOTE:
>
> the man page seems to imply that the current behaviour (-EAGAIN on allocation
> failure) has always been the case. I don't think it makes a lot of sense, but
> this should probably be discussed more widely in case applications have somehow
> come to rely on the current behaviour...

We can't really feasibly fix this, is my worry. Fio does track the 
pending requests and does not get into a getevents() forever wait if it 
gets -EAGAIN on submission. But before the fix, it would loop forever in 
submission in -EAGAIN.

How are applications supposed to deal with ENOMEM? I think the answer 
here is that they can't, it would be a fatal condition. AIO must provide 
isn't own guarantee of progress, with a mempool or similar.


-- 
Jens Axboe

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