Messages in this thread | | | Date | Fri, 03 Oct 2014 12:36:10 -0600 | From | Jens Axboe <> | Subject | Re: [PATCH] aio: Fix return code of io_submit() (RFC) |
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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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