splice: __generic_file_splice_read: fix read/truncate race

Original patch and description from Neil Brown <neilb@suse.de>,
merged and adapted to splice branch by me. Neils text follows:

__generic_file_splice_read() currently samples the i_size at the start
and doesn't do so again unless it needs to call ->readpage to load
a page.  After ->readpage it has to re-sample i_size as a truncate
may have caused that page to be filled with zeros, and the read()
call should not see these.

However there are other activities that might cause ->readpage to be
called on a page between the time that __generic_file_splice_read()
samples i_size and when it finds that it has an uptodate page. These
include at least read-ahead and possibly another thread performing a
read

So we must sample i_size *after* it has an uptodate page.  Thus the
current sampling at the start and after a read can be replaced with a
sampling before page addition into spd.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/splice.c

@@ -413,37 +413,37 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
 
 				break;
 			}
+		}
+fill_it:
+		/*
+		 * i_size must be checked after PageUptodate.
+		 */
+		isize = i_size_read(mapping->host);
+		end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
+		if (unlikely(!isize || index > end_index))
+			break;
+
+		/*
+		 * if this is the last page, see if we need to shrink
+		 * the length and stop
+		 */
+		if (end_index == index) {
+			unsigned int plen;
 
 			/*
-			 * i_size must be checked after ->readpage().
+			 * max good bytes in this page
 			 */
-			isize = i_size_read(mapping->host);
-			end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
-			if (unlikely(!isize || index > end_index))
+			plen = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
+			if (plen <= loff)
 				break;
 
 			/*
-			 * if this is the last page, see if we need to shrink
-			 * the length and stop
+			 * force quit after adding this page
 			 */
-			if (end_index == index) {
-				unsigned int plen;
-
-				/*
-				 * max good bytes in this page
-				 */
-				plen = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
-				if (plen <= loff)
-					break;
-
-				/*
-				 * force quit after adding this page
-				 */
-				this_len = min(this_len, plen - loff);
-				len = this_len;
-			}
+			this_len = min(this_len, plen - loff);
+			len = this_len;
 		}
-fill_it:
+
 		partial[page_nr].offset = loff;
 		partial[page_nr].len = this_len;
 		len -= this_len;