Messages in this thread | | | Date | Fri, 3 Oct 2014 11:21:51 -0700 | From | Kent Overstreet <> | Subject | Re: [PATCH] aio: Fix return code of io_submit() (RFC) |
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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.
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