UBI: reduce memory consumption

Remove the pre-allocated 'peb_buf2' buffer because we do not really need it.
The only reason UBI has it is to check that the data were written correctly.
But we do not have to have 2 buffers for this and waste RAM - we can just
compare CRC checksums instead. This reduces UBI memory consumption.

Artem bityutskiy: massaged the patch and commit message
Signed-off-by: Josselin Costanzi <josselin.costanzi@mobile-devices.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/build.c

@@ -949,10 +949,6 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num, int vid_hdr_offset)
 	if (!ubi->peb_buf1)
 		goto out_free;
 
-	ubi->peb_buf2 = vmalloc(ubi->peb_size);
-	if (!ubi->peb_buf2)
-		goto out_free;
-
 	err = ubi_debugging_init_dev(ubi);
 	if (err)
 		goto out_free;
@@ -1030,7 +1026,6 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num, int vid_hdr_offset)
 	ubi_debugging_exit_dev(ubi);
 out_free:
 	vfree(ubi->peb_buf1);
-	vfree(ubi->peb_buf2);
 	if (ref)
 		put_device(&ubi->dev);
 	else
@@ -1102,7 +1097,6 @@ int ubi_detach_mtd_dev(int ubi_num, int anyway)
 	put_mtd_device(ubi->mtd);
 	ubi_debugging_exit_dev(ubi);
 	vfree(ubi->peb_buf1);
-	vfree(ubi->peb_buf2);
 	ubi_msg("mtd%d is detached from ubi%d", ubi->mtd->index, ubi->ubi_num);
 	put_device(&ubi->dev);
 	return 0;

drivers/mtd/ubi/eba.c

@@ -1134,8 +1134,8 @@ int ubi_eba_copy_leb(struct ubi_device *ubi, int from, int to,
 		 * We've written the data and are going to read it back to make
 		 * sure it was written correctly.
 		 */
-
-		err = ubi_io_read_data(ubi, ubi->peb_buf2, to, 0, aldata_size);
+		memset(ubi->peb_buf1, 0xFF, aldata_size);
+		err = ubi_io_read_data(ubi, ubi->peb_buf1, to, 0, aldata_size);
 		if (err) {
 			if (err != UBI_IO_BITFLIPS) {
 				ubi_warn("error %d while reading data back "
@@ -1149,7 +1149,7 @@ int ubi_eba_copy_leb(struct ubi_device *ubi, int from, int to,
 
 		cond_resched();
 
-		if (memcmp(ubi->peb_buf1, ubi->peb_buf2, aldata_size)) {
+		if (crc != crc32(UBI_CRC32_INIT, ubi->peb_buf1, data_size)) {
 			ubi_warn("read data back from PEB %d and it is "
 				 "different", to);
 			err = -EINVAL;

drivers/mtd/ubi/ubi.h

@@ -388,8 +388,7 @@ struct ubi_wl_entry;
  * @mtd: MTD device descriptor
  *
  * @peb_buf1: a buffer of PEB size used for different purposes
- * @peb_buf2: another buffer of PEB size used for different purposes
- * @buf_mutex: protects @peb_buf1 and @peb_buf2
+ * @buf_mutex: protects @peb_buf1
  * @ckvol_mutex: serializes static volume checking when opening
  *
  * @dbg: debugging information for this UBI device
@@ -472,7 +471,6 @@ struct ubi_device {
 	struct mtd_info *mtd;
 
 	void *peb_buf1;
-	void *peb_buf2;
 	struct mutex buf_mutex;
 	struct mutex ckvol_mutex;