ext4: Use page_mkwrite vma_operations to get mmap write notification.

We would like to get notified when we are doing a write on mmap section.
This is needed with respect to preallocated area. We split the preallocated
area into initialzed extent and uninitialzed extent in the call back. This
let us handle ENOSPC better. Otherwise we get ENOSPC in the writepage and
that would result in data loss. The changes are also needed to handle ENOSPC
when writing to an mmap section of files with holes.

Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 61 insertions, 0 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index c5d506dce8c5..034fc544aa66 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3564,3 +3564,64 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
 
         return err;
 }
+
+static int ext4_bh_unmapped(handle_t *handle, struct buffer_head *bh)
+{
+        return !buffer_mapped(bh);
+}
+
+int ext4_page_mkwrite(struct vm_area_struct *vma, struct page *page)
+{
+        loff_t size;
+        unsigned long len;
+        int ret = -EINVAL;
+        struct file *file = vma->vm_file;
+        struct inode *inode = file->f_path.dentry->d_inode;
+        struct address_space *mapping = inode->i_mapping;
+
+        /*
+         * Get i_alloc_sem to stop truncates messing with the inode. We cannot
+         * get i_mutex because we are already holding mmap_sem.
+         */
+        down_read(&inode->i_alloc_sem);
+        size = i_size_read(inode);
+        if (page->mapping != mapping || size <= page_offset(page)
+            || !PageUptodate(page)) {
+                /* page got truncated from under us? */
+                goto out_unlock;
+        }
+        ret = 0;
+        if (PageMappedToDisk(page))
+                goto out_unlock;
+
+        if (page->index == size >> PAGE_CACHE_SHIFT)
+                len = size & ~PAGE_CACHE_MASK;
+        else
+                len = PAGE_CACHE_SIZE;
+
+        if (page_has_buffers(page)) {
+                /* return if we have all the buffers mapped */
+                if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
+                                       ext4_bh_unmapped))
+                        goto out_unlock;
+        }
+        /*
+         * OK, we need to fill the hole... Do write_begin write_end
+         * to do block allocation/reservation.We are not holding
+         * inode.i__mutex here. That allow * parallel write_begin,
+         * write_end call. lock_page prevent this from happening
+         * on the same page though
+         */
+        ret = mapping->a_ops->write_begin(file, mapping, page_offset(page),
+                        len, AOP_FLAG_UNINTERRUPTIBLE, &page, NULL);
+        if (ret < 0)
+                goto out_unlock;
+        ret = mapping->a_ops->write_end(file, mapping, page_offset(page),
+                        len, len, page, NULL);
+        if (ret < 0)
+                goto out_unlock;
+        ret = 0;
+out_unlock:
+        up_read(&inode->i_alloc_sem);
+        return ret;
+}