do_generic_file_read: s/EINTR/EIO/ if lock_page_killable() fails

If lock_page_killable() fails because the task was killed by SIGKILL or
any other fatal signal, do_generic_file_read() returns -EIO.

This seems to be OK, because in fact the userspace won't see this error,
the task will dequeue SIGKILL and exit.

However, /sbin/init is different, it will dequeue SIGKILL, ignore it, and
return to the user-space with the bogus -EIO.

Change the code to return the error code from lock_page_killable(), -EINTR.
This doesn't fix the bug, but perhaps makes sense anyway. Imho, with this
change the code looks a bit more logical, and the "good" init should handle
the spurious EINTR or short read.

Afaics we can also change lock_page_killable() to return -ERESTARTNOINTR,
but this can't prevent the short reads.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

mm/filemap.c

@@ -1100,8 +1100,9 @@ static void do_generic_file_read(struct file *filp, loff_t *ppos,
 
 page_not_up_to_date:
 		/* Get exclusive access to the page ... */
-		if (lock_page_killable(page))
-			goto readpage_eio;
+		error = lock_page_killable(page);
+		if (unlikely(error))
+			goto readpage_error;
 
 page_not_up_to_date_locked:
 		/* Did it get truncated before we got the lock? */
@@ -1130,8 +1131,9 @@ static void do_generic_file_read(struct file *filp, loff_t *ppos,
 		}
 
 		if (!PageUptodate(page)) {
-			if (lock_page_killable(page))
-				goto readpage_eio;
+			error = lock_page_killable(page);
+			if (unlikely(error))
+				goto readpage_error;
 			if (!PageUptodate(page)) {
 				if (page->mapping == NULL) {
 					/*
@@ -1143,15 +1145,14 @@ static void do_generic_file_read(struct file *filp, loff_t *ppos,
 				}
 				unlock_page(page);
 				shrink_readahead_size_eio(filp, ra);
-				goto readpage_eio;
+				error = -EIO;
+				goto readpage_error;
 			}
 			unlock_page(page);
 		}
 
 		goto page_ok;
 
-readpage_eio:
-		error = -EIO;
 readpage_error:
 		/* UHHUH! A synchronous read error occurred. Report it */
 		desc->error = error;