pipe(2) already documents this flag. Since the following commit changes to O_DIRECT via fcntl are allowed too: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/fcntl.c?id=0dbf5f20652108106cb822ad7662c786baaa03ff
I've added this sentence to pipe(2): Since Linux 4.5, it is possible to change the O_DIRECT set‐ ting of a pipe file descriptor using fcntl(2). I'll close this bug now. Please reopen if you think more work is needed.