[PATCH] filemap read() fix

Fix the do_generic_file_read()-reads-one-page-too-many-bug for the fifth
time.

This patch combines the best features from Nick's patch and also makes
index and end_index consistent.  (i.e index 'n' covers n*PAGE_SIZE to
((n+1)PAGE_SIZE)-1.  I did not feel comfortable with the way index and
end_index represented different ranges.  It was like comparing apples with
oranges.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/filemap.c

@@ -722,7 +722,10 @@ void do_generic_mapping_read(struct address_space *mapping,
 	offset = *ppos & ~PAGE_CACHE_MASK;
 
 	isize = i_size_read(inode);
-	end_index = isize >> PAGE_CACHE_SHIFT;
+	if (!isize)
+		goto out;
+
+	end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
 	if (index > end_index)
 		goto out;
 
@@ -730,6 +733,16 @@ void do_generic_mapping_read(struct address_space *mapping,
 		struct page *page;
 		unsigned long nr, ret;
 
+		/* nr is the maximum number of bytes to copy from this page */
+		nr = PAGE_CACHE_SIZE;
+		if (index == end_index) {
+			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
+			if (nr <= offset) {
+				goto out;
+			}
+		}
+		nr = nr - offset;
+
 		cond_resched();
 		page_cache_readahead(mapping, &ra, filp, index);
 
@@ -742,16 +755,6 @@ void do_generic_mapping_read(struct address_space *mapping,
 		if (!PageUptodate(page))
 			goto page_not_up_to_date;
 page_ok:
-		/* nr is the maximum number of bytes to copy from this page */
-		nr = PAGE_CACHE_SIZE;
-		if (index == end_index) {
-			nr = isize & ~PAGE_CACHE_MASK;
-			if (nr <= offset) {
-				page_cache_release(page);
-				goto out;
-			}
-		}
-		nr = nr - offset;
 
 		/* If users can be writing to this page using arbitrary
 		 * virtual addresses, take care about potential aliasing
@@ -827,11 +830,22 @@ void do_generic_mapping_read(struct address_space *mapping,
 		 * another truncate extends the file - this is desired though).
 		 */
 		isize = i_size_read(inode);
-		end_index = isize >> PAGE_CACHE_SHIFT;
-		if (index > end_index) {
+		end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
+		if (unlikely(!isize || index > end_index)) {
 			page_cache_release(page);
 			goto out;
 		}
+
+		/* nr is the maximum number of bytes to copy from this page */
+		nr = PAGE_CACHE_SIZE;
+		if (index == end_index) {
+			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
+			if (nr <= offset) {
+				page_cache_release(page);
+				goto out;
+			}
+		}
+		nr = nr - offset;
 		goto page_ok;
 
 readpage_error: