Re: [PATCH] aio: Fix return code of io_submit() (RFC)

[Jens Axboe]

On 2014-10-03 12:22, Benjamin LaHaise wrote:
> On Fri, Oct 03, 2014 at 12:13:39PM -0600, Jens Axboe wrote:
> > 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.
>
> There are lots of instances in the kernel where out of memory is potentially
> exposed to the user.  If we're failing a memory allocation that is well under
> 1KB, the system is probably completely hosed.
>
> > 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.

Since fio just hit this, it has nothing it can potentially free to make 
progress possible. There was no pending IO, so all it can do is quit. 
But I do agree that if a small alloc like that fails, then we are 
probably pretty darn screwed anyway, and it doesn't matter that much 
what we do. My main concern was a potential change in the ABI, but since 
we could already return -ENOMEM from other cases, that is probably a 
moot point.

--
Jens Axboe

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