filemap: add a comment about FAULT_FLAG_RETRY_NOWAIT behavior

I thought Josef Bacik's patch to drop the mmap_sem was buggy, because
when looking at the error cases, there was one case where we returned
VM_FAULT_RETRY without actually dropping the mmap_sem.

Josef had to explain to me (using small words) that yes, that's actually
what we're supposed to do, and his patch was correct.  Which not only
convinced me he knew what he was doing and I should stop arguing with
him, but also that I should add a comment to the case I was confused
about.

Patiently-pointed-out-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2428,6 +2428,11 @@ static int lock_page_maybe_drop_mmap(struct vm_fault *vmf, struct page *page,
 	if (trylock_page(page))
 		return 1;
 
+	/*
+	 * NOTE! This will make us return with VM_FAULT_RETRY, but with
+	 * the mmap_sem still held. That's how FAULT_FLAG_RETRY_NOWAIT
+	 * is supposed to work. We have way too many special cases..
+	 */
 	if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
 		return 0;