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

[Jens Axboe <axboe@kernel.dk>]

On 2014-10-03 12:21, Kent Overstreet 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.
>>
>> 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".

It'll block anyway in other places, if we run out of resources there. 
But good point on the other potential -ENOMEM cases, it's not a new 
condition (potentially).

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

Me neither...

-- 
Jens Axboe