mm: new pfn_mkwrite same as page_mkwrite for VM_PFNMAP

This will allow FS that uses VM_PFNMAP | VM_MIXEDMAP (no page structs) to
get notified when access is a write to a read-only PFN.

This can happen if we mmap() a file then first mmap-read from it to
page-in a read-only PFN, than we mmap-write to the same page.

We need this functionality to fix a DAX bug, where in the scenario above
we fail to set ctime/mtime though we modified the file.  An xfstest is
attached to this patchset that shows the failure and the fix.  (A DAX
patch will follow)

This functionality is extra important for us, because upon dirtying of a
pmem page we also want to RDMA the page to a remote cluster node.

We define a new pfn_mkwrite and do not reuse page_mkwrite because
  1 - The name ;-)
  2 - But mainly because it would take a very long and tedious
      audit of all page_mkwrite functions of VM_MIXEDMAP/VM_PFNMAP
      users. To make sure they do not now CRASH. For example current
      DAX code (which this is for) would crash.
      If we would want to reuse page_mkwrite, We will need to first
      patch all users, so to not-crash-on-no-page. Then enable this
      patch. But even if I did that I would not sleep so well at night.
      Adding a new vector is the safest thing to do, and is not that
      expensive. an extra pointer at a static function vector per driver.
      Also the new vector is better for performance, because else we
      Will call all current Kernel vectors, so to:
        check-ha-no-page-do-nothing and return.

No need to call it from do_shared_fault because do_wp_page is called to
change pte permissions anyway.

Signed-off-by: Yigal Korman <yigal@plexistor.com>
Signed-off-by: Boaz Harrosh <boaz@plexistor.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index f91926f2f482..8bb8a7ee0f99 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -525,6 +525,7 @@ prototypes:
         void (*close)(struct vm_area_struct*);
         int (*fault)(struct vm_area_struct*, struct vm_fault *);
         int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
+        int (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
         int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
 
 locking rules:
@@ -534,6 +535,7 @@ close:		yes
 fault:          yes             can return with page locked
 map_pages:      yes
 page_mkwrite:   yes             can return with page locked
+pfn_mkwrite:    yes
 access:         yes
 
         ->fault() is called when a previously not present pte is about
@@ -560,6 +562,12 @@ the page has been truncated, the filesystem should not look up a new page
 like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
 will cause the VM to retry the fault.
 
+        ->pfn_mkwrite() is the same as page_mkwrite but when the pte is
+VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
+VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
+after this call is to make the pte read-write, unless pfn_mkwrite returns
+an error.
+
         ->access() is called when get_user_pages() fails in
 access_process_vm(), typically used to debug a process through
 /proc/pid/mem or ptrace.  This function is needed only for