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

[Kent Overstreet <kmo@daterainc.com>]

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.
> 
> 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.

Well, even though the AIO code doesn't currently return -ENOMEM we definitely do
have random other driver/filesystem code that will return -ENOMEM if a random
GFP_KERNEL allocation fails (e.g. the dio code, if allocating a struct dio
fails). So I think there's precedent for this, and having it be a fatal error
when the system is under major memory pressure is not a crazy thing to do too.

But OTOH maybe we should just use a mempool there.

The argument against making it a mempool would be "we don't want io_submit() to
block; even if that's not the case today, we at least have a chance of fixing it
with the current setup. If we can't allocate memory for our asynchronous state,
we really can't do anything there except block or fail".

I'm not sure I have strong feelings one way or the other.