Document ->page_mkwrite() locking

There seems to be very little documentation about this callback in general.
The locking in particular is a bit tricky, so it's worth having this in
writing.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/Locking

@@ -512,13 +512,22 @@ prototypes:
 	void (*close)(struct vm_area_struct*);
 	struct page *(*fault)(struct vm_area_struct*, struct fault_data *);
 	struct page *(*nopage)(struct vm_area_struct*, unsigned long, int *);
+	int (*page_mkwrite)(struct vm_area_struct *, struct page *);
 
 locking rules:
-		BKL	mmap_sem
+		BKL	mmap_sem	PageLocked(page)
 open:		no	yes
 close:		no	yes
 fault:		no	yes
 nopage:		no	yes
+page_mkwrite:	no	yes		no
+
+	->page_mkwrite() is called when a previously read-only page is
+about to become writeable. The file system is responsible for
+protecting against truncate races. Once appropriate action has been
+taking to lock out truncate, the page range should be verified to be
+within i_size. The page mapping should also be checked that it is not
+NULL.
 
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 			Dubious stuff