On Thu, Nov 7, 2019 at 5:39 AM David Howells [off-list ref] wrote:

Add an O_NOTIFICATION_PIPE flag that can be passed to pipe2() to indicate
that the pipe being created is going to be used for notifications.  This
suppresses the use of splice(), vmsplice(), tee() and sendfile() on the
pipe as calling iov_iter_revert() on a pipe when a kernel notification
message has been inserted into the middle of a multi-buffer splice will be
messy.

How messy?  And is there some way to make it impossible for this to
happen?  Adding a new flag to pipe2() to avoid messy kernel code seems
like a poor tradeoff.