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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
