I would like to tell if a process has opened any files using O_DIRECT, but I can only examine it after the process was launched (i.e. strace is not an option). I tried looking in /proc/$pid/fd/ to see if there was anything useful, but there wasn’t. My goal is to track down if any of several hundred users on a system have opened files with O_DIRECT. Is this possible?
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Since kernel 2.6.22, /proc/$pid/fdinfo/$fd contains a flags field, in octal. See http://www.kernel.org/doc/man-pages/online/pages/man5/proc.5.html