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

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Date:	Fri, 3 Oct 2014 11:31:47 -0700
From:	Kent Overstreet <kmo@...erainc.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Jens Axboe <axboe@...nel.dk>, 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 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).

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".

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