splice: don't read more than available pipe space
In commit 4721a601, we tried to fix a problem wherein directio reads into a splice pipe will bounce EFAULT/EAGAIN all the way out to userspace by simulating a zero-byte short read. This happens because some directio read implementations (xfs) will call bio_iov_iter_get_pages to grab pipe buffer pages and issue asynchronous reads, but as soon as we run out of pipe buffers that _get_pages call returns EFAULT, which the splice code translates to EAGAIN and bounces out to userspace. In that commit, the iomap code catches the EFAULT and simulates a zero-byte read, but that causes assertion errors on regular splice reads because xfs doesn't allow short directio reads. The brokenness is compounded by splice_direct_to_actor immediately bailing on do_splice_to returning <= 0 without ever calling ->actor (which empties out the pipe), so if userspace calls back we'll EFAULT again on the full pipe, and nothing ever gets copied. Therefore, teach splice_direct_to_actor to clamp its requests to the amount of free space in the pipe and remove the simulated short read from the iomap directio code. Fixes: 4721a601 ("iomap: dio data corruption and spurious errors when pipes fill") Reported-by:Murphy Zhou <jencce.kernel@gmail.com> Ranted-by: