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

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Date:	Fri, 03 Oct 2014 12:39:20 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Kent Overstreet <kmo@...erainc.com>,
	Benjamin LaHaise <bcrl@...ck.org>
CC:	linux-kernel@...r.kernel.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 12:31, Kent Overstreet wrote:
> On Fri, Oct 03, 2014 at 02:22:20PM -0400, Benjamin LaHaise wrote:
>> On Fri, Oct 03, 2014 at 12:13:39PM -0600, Jens Axboe wrote:
>>> 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.
>>
>> I'm not sure if using a mempool is appropriate for allocations that are
>> driven by userland code.  At least with an ENOMEM error, an application
>> could free up some of the memory it allocated and possibly recover the
>> system.
>
> I guess it's going to depend on the application... some applications really want
> to always make forward progress (much like a lot of code in the kernel), so
> they're going to want the mempool semantics and we in the kernel are in a much
> better position to implement that correctly (think of all the applications that
> are just going to sleep and retry on -ENOMEM).

Precisely, there's no real way to do that in the application. Especially 
if it has no pending IO it can just wait on, it'll be a sleep and retry 
thing

> we kind of want another flag in the syscall args that's the moral equivalent of
> MSG_DONTWAIT but for memory allocations; it'd translate into "mempool +
> GFP_KERNEL, or GFP_NOWAIT".

We do...

> not that I'm actually going to implement that :)

It's worth keeping in mind for if we do extend the API for some reason.

-- 
Jens Axboe

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