so an error on the 1st page gets propagated to the caller, and that get_user_pages_unlocked eventually calls __get_user_pages so it does return an error sometimes.
Would it be correct to apply the second part of the patch then (pasted below for reference) or should get_user_pages_fast and all its callers be changed to return 0 on error instead?
0 isn't an error. As SuS sees it (ie from the userspace end of the pile)
returning the number you asked for means it worked
returning a smaller number means it worked partially and that much was consumed (or in some cases more and the rest if so was lost - depends what you are reading/writing)
returning 0 means you read nothing as you were at the end of file
returning an error code means it broke, or you should try again (EAGAIN/EWOULDBLOCK)
The ugly bit there is the try-again semantics needs to exactly match the attached poll() behaviour or you get busy loops.
Alan