On 12/17/20 1:19 AM, Dmitry Kadashev wrote: > Hi, > > We've ran into something that looks like a memory accounting problem > in the kernel / io_uring code. We use multiple rings per process, and > generally it works fine. Until it does not - new ring creation just > fails with ENOMEM. And at that point it fails consistently until the > box is rebooted. > > More details: we use multiple rings per process, typically they are > initialized on the process start (not necessarily, but that is not > important here, let's just assume all are initialized on the process > start). On a freshly booted box everything works fine. But after a > while - and some process restarts - io_uring_queue_init() starts to > fail with ENOMEM. Sometimes we see it fail, but then subsequent ones > succeed (in the same process), but over time it gets worse, and > eventually no ring can be initialized. And once that happens the only > way to fix the problem is to restart the box. Most of the mentioned > restarts are graceful: a new process is started and then the old one > is killed, possibly with the KILL signal if it does not shut down in > time. Things work fine for some time, but eventually we start getting > those errors. > > Originally we've used 5.6.6 kernel, but given the fact quite a few > accounting issues were fixed in io_uring in 5.8, we've tried 5.9.5 as > well, but the issue is not gone. > > Just in case, everything else seems to be working fine, it just falls > back to the thread pool instead of io_uring, and then everything > continues to work just fine. > > I was not able to spot anything suspicious in the /proc/meminfo. We > have RLIMIT_MEMLOCK set to infinity. And on a box that currently > experiences the problem /proc/meminfo shows just 24MB as locked. > > Any pointers to how can we debug this? I've read through this thread, but haven't had time to really debug it yet. I did try a few test cases, and wasn't able to trigger anything. The signal part is interesting, as it would cause parallel teardowns potentially. And I did post a patch for that yesterday, where I did spot a race in the user mm accounting. I don't think this is related to this one, but would still be useful if you could test with this applied: https://lore.kernel.org/io-uring/20201217152105.693264-3-axboe@xxxxxxxxx/T/#u just in case... -- Jens Axboe
