[PATCH 3/3] mm: Provide stub page_mkwrite functionality to stabilize pages during writes

["Darrick J. Wong" <djwong@us.ibm.com>]

For filesystems that do not provide any page_mkwrite handler, provide a stub
page_mkwrite function that locks the page and waits for pending writeback to
complete.  This is needed to stabilize pages during writes for a large variety
of filesystem drivers (ext2, ext3, vfat, hfs...).

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
---
 mm/filemap.c |   19 +++++++++++++++++++
 1 files changed, 19 insertions(+), 0 deletions(-)


diff --git a/mm/filemap.c b/mm/filemap.c
index fd0e7f2..2a922b4 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1713,8 +1713,27 @@ page_not_uptodate:
 }
 EXPORT_SYMBOL(filemap_fault);
 
+static int stub_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	struct page *page = vmf->page;
+	struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
+	loff_t size;
+
+	lock_page(page);
+	size = i_size_read(inode);
+	if ((page->mapping != inode->i_mapping) ||
+	    (page_offset(page) > size)) {
+		/* page got truncated out from underneath us */
+		unlock_page(page);
+		return VM_FAULT_NOPAGE;
+	}
+	wait_on_page_writeback(page);
+	return VM_FAULT_LOCKED;
+}
+
 const struct vm_operations_struct generic_file_vm_ops = {
 	.fault		= filemap_fault,
+	.page_mkwrite	= stub_page_mkwrite,
 };
 
 /* This is used for a general mmap of a disk file */

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