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, 39 insertions, 4 deletions

diff --git a/mm/memory.c b/mm/memory.c
index f9628e5..22e037e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2180,6 +2180,42 @@ oom:
 	return VM_FAULT_OOM;
 }
 
+/*
+ * Handle write page faults for VM_MIXEDMAP or VM_PFNMAP for a VM_SHARED
+ * mapping
+ */
+static int wp_pfn_shared(struct mm_struct *mm,
+			struct vm_area_struct *vma, unsigned long address,
+			pte_t *page_table, spinlock_t *ptl, pte_t orig_pte,
+			pmd_t *pmd)
+{
+	if (vma->vm_ops && vma->vm_ops->pfn_mkwrite) {
+		struct vm_fault vmf = {
+			.page = NULL,
+			.pgoff = linear_page_index(vma, address),
+			.virtual_address = (void __user *)(address & PAGE_MASK),
+			.flags = FAULT_FLAG_WRITE | FAULT_FLAG_MKWRITE,
+		};
+		int ret;
+
+		pte_unmap_unlock(page_table, ptl);
+		ret = vma->vm_ops->pfn_mkwrite(vma, &vmf);
+		if (ret & VM_FAULT_ERROR)
+			return ret;
+		page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
+		/*
+		 * We might have raced with another page fault while we
+		 * released the pte_offset_map_lock.
+		 */
+		if (!pte_same(*page_table, orig_pte)) {
+			pte_unmap_unlock(page_table, ptl);
+			return 0;
+		}
+	}
+	return wp_page_reuse(mm, vma, address, page_table, ptl, orig_pte,
+			     NULL, 0, 0);
+}
+
 static int wp_page_shared(struct mm_struct *mm, struct vm_area_struct *vma,
 			  unsigned long address, pte_t *page_table,
 			  pmd_t *pmd, spinlock_t *ptl, pte_t orig_pte,
@@ -2258,13 +2294,12 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		 * VM_PFNMAP VMA.
 		 *
 		 * We should not cow pages in a shared writeable mapping.
-		 * Just mark the pages writable as we can't do any dirty
-		 * accounting on raw pfn maps.
+		 * Just mark the pages writable and/or call ops->pfn_mkwrite.
 		 */
 		if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
 				     (VM_WRITE|VM_SHARED))
-			return wp_page_reuse(mm, vma, address, page_table, ptl,
-					     orig_pte, old_page, 0, 0);
+			return wp_pfn_shared(mm, vma, address, page_table, ptl,
+					     orig_pte, pmd);
 
 		pte_unmap_unlock(page_table, ptl);
 		return wp_page_copy(mm, vma, address, page_table, pmd,