md: don't write more than is required of the last page of a bitmap

It is possible that real data or metadata follows the bitmap without full page
alignment.

So limit the last write to be only the required number of bytes, rounded up to
the hard sector size of the device.
Signed-off-by: Neil Brown <neilb@suse.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/bitmap.c

@@ -255,19 +255,25 @@ static struct page *read_sb_page(mddev_t *mddev, long offset, unsigned long inde
 
 }
 
-static int write_sb_page(mddev_t *mddev, long offset, struct page *page, int wait)
+static int write_sb_page(struct bitmap *bitmap, struct page *page, int wait)
 {
 	mdk_rdev_t *rdev;
 	struct list_head *tmp;
+	mddev_t *mddev = bitmap->mddev;
 
 	ITERATE_RDEV(mddev, rdev, tmp)
 		if (test_bit(In_sync, &rdev->flags)
-		    && !test_bit(Faulty, &rdev->flags))
+		    && !test_bit(Faulty, &rdev->flags)) {
+			int size = PAGE_SIZE;
+			if (page->index == bitmap->file_pages-1)
+				size = roundup(bitmap->last_page_size,
+					       bdev_hardsect_size(rdev->bdev));
 			md_super_write(mddev, rdev,
-				       (rdev->sb_offset<<1) + offset
+				       (rdev->sb_offset<<1) + bitmap->offset
 				       + page->index * (PAGE_SIZE/512),
-				       PAGE_SIZE,
+				       size,
 				       page);
+		}
 
 	if (wait)
 		md_super_wait(mddev);
@@ -282,7 +288,7 @@ static int write_page(struct bitmap *bitmap, struct page *page, int wait)
 	struct buffer_head *bh;
 
 	if (bitmap->file == NULL)
-		return write_sb_page(bitmap->mddev, bitmap->offset, page, wait);
+		return write_sb_page(bitmap, page, wait);
 
 	bh = page_buffers(page);
 
@@ -923,6 +929,7 @@ static int bitmap_init_from_disk(struct bitmap *bitmap, sector_t start)
 			}
 
 			bitmap->filemap[bitmap->file_pages++] = page;
+			bitmap->last_page_size = count;
 		}
 		paddr = kmap_atomic(page, KM_USER0);
 		if (bitmap->flags & BITMAP_HOSTENDIAN)

include/linux/raid/bitmap.h

@@ -232,6 +232,7 @@ struct bitmap {
 	struct page **filemap; /* list of cache pages for the file */
 	unsigned long *filemap_attr; /* attributes associated w/ filemap pages */
 	unsigned long file_pages; /* number of pages in the file */
+	int last_page_size; /* bytes in the last page */
 
 	unsigned long flags;