Explain -ENOMEM return value from io_uring_queue_init()

io_uring_queue_init() can return -ENOMEM for reasons that are sensible
but not intuitively obvious.  Clarify what's happening and what the user
should do to fix it.

Closes: #1560
Signed-off-by: Andrew Sayers <andrew-github.com@pileofstuff.org>

Author: andrew-sayers

man/io_uring_queue_exit.3

@@ -18,6 +18,10 @@ io_uring_queue_exit \- tear down io_uring submission and completion queues
 will release all resources acquired and initialized by
 .BR io_uring_queue_init (3).
 It first unmaps the memory shared between the application and the kernel and then closes the io_uring file descriptor.
+
+Some accounting is done asynchronously, so memory locked by
+.BR io_uring_queue_init (3)
+may remain locked for a few milliseconds after this function returns.
 .SH RETURN VALUE
 None
 .SH SEE ALSO

man/io_uring_queue_init.3

@@ -118,6 +118,20 @@ and
 return 0 on success and
 .BR -errno
 on failure.
+A return value of
+.BR -ENOMEM
+indicates there is not enough locked memory available to hold the specified
+number of entries.
+Reduce the number of entries, or call
+.BR setrlimit (2)
+to increase the maximum number of bytes of memory that may be locked into RAM.
+Be aware that calling
+.BR io_uring_queue_init (3)
+and
+.BR io_uring_queue_exit (3)
+in a loop will temporarily lock a lot of memory, because
+.BR io_uring_queue_exit (3)
+does some of its accounting asynchronously.
 
 .BR io_uring_queue_init_mem (3)
 returns the number of bytes used from the provided buffer on success, and