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>

1 files changed, 10 insertions, 1 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 970c8ec1a05..91ec4b40ebf 100644
--- a/Documentation/filesystems/Locking
+++ b/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