pipe: change pipe_write() to never add a zero-sized buffer

a194dfe6 ("pipe: Rearrange sequence in pipe_write() to preallocate slot")
changed pipe_write() to increment pipe->head in advance.  IIUC to avoid the
race with the post_one_notification()-like code which can add another buffer
under pipe->rd_wait.lock without pipe->mutex.

This is no longer necessary after c73be61c ("pipe: Add general notification
queue support"), pipe_write() checks pipe_has_watch_queue() and returns -EXDEV
at the start. And can't help in any case, pipe_write() no longer takes this
rd_wait.lock spinlock.

Change pipe_write() to call copy_page_from_iter() first and do nothing if it
fails. This way pipe_write() can't add a zero-sized buffer and we can simplify
pipe_read() which currently has to take care of this very unlikely case.

Also, with this patch we can probably kill eat_empty_buffer() and more
"is this buffer empty" checks in fs/splice.c later.

Link: https://lore.kernel.org/all/20250209150718.GA17013@redhat.com/


Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20250210114039.GA3588@redhat.com


Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>