fat: don't use free_clusters for fat32

It seems that the recent Windows changed specification, and it's
undocumented.  Windows doesn't update ->free_clusters correctly.

This patch doesn't use ->free_clusters by default.  (instead, add "usefree"
for forcing to use it)

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Juergen Beisert <juergen127@kreuzholzen.de>
Cc: Andreas Schwab <schwab@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/vfat.txt

@@ -57,6 +57,13 @@ nonumtail=<bool> -- When creating 8.3 aliases, normally the alias will
                  currently exist in the directory, 'longfile.txt' will
                  be the short alias instead of 'longfi~1.txt'. 
                   
+usefree       -- Use the "free clusters" value stored on FSINFO. It'll
+                 be used to determine number of free clusters without
+                 scanning disk. But it's not used by default, because
+                 recent Windows don't update it correctly in some
+                 case. If you are sure the "free clusters" on FSINFO is
+                 correct, by this option you can avoid scanning disk.
+
 quiet         -- Stops printing certain warning messages.
 
 check=s|r|n   -- Case sensitivity checking setting.

fs/fat/inode.c

@@ -825,6 +825,8 @@ static int fat_show_options(struct seq_file *m, struct vfsmount *mnt)
 	}
 	if (opts->name_check != 'n')
 		seq_printf(m, ",check=%c", opts->name_check);
+	if (opts->usefree)
+		seq_puts(m, ",usefree");
 	if (opts->quiet)
 		seq_puts(m, ",quiet");
 	if (opts->showexec)
@@ -850,7 +852,7 @@ static int fat_show_options(struct seq_file *m, struct vfsmount *mnt)
 
 enum {
 	Opt_check_n, Opt_check_r, Opt_check_s, Opt_uid, Opt_gid,
-	Opt_umask, Opt_dmask, Opt_fmask, Opt_codepage, Opt_nocase,
+	Opt_umask, Opt_dmask, Opt_fmask, Opt_codepage, Opt_usefree, Opt_nocase,
 	Opt_quiet, Opt_showexec, Opt_debug, Opt_immutable,
 	Opt_dots, Opt_nodots,
 	Opt_charset, Opt_shortname_lower, Opt_shortname_win95,
@@ -872,6 +874,7 @@ static match_table_t fat_tokens = {
 	{Opt_dmask, "dmask=%o"},
 	{Opt_fmask, "fmask=%o"},
 	{Opt_codepage, "codepage=%u"},
+	{Opt_usefree, "usefree"},
 	{Opt_nocase, "nocase"},
 	{Opt_quiet, "quiet"},
 	{Opt_showexec, "showexec"},
@@ -951,7 +954,7 @@ static int parse_options(char *options, int is_vfat, int silent, int *debug,
 	opts->quiet = opts->showexec = opts->sys_immutable = opts->dotsOK =  0;
 	opts->utf8 = opts->unicode_xlate = 0;
 	opts->numtail = 1;
-	opts->nocase = 0;
+	opts->usefree = opts->nocase = 0;
 	*debug = 0;
 
 	if (!options)
@@ -979,6 +982,9 @@ static int parse_options(char *options, int is_vfat, int silent, int *debug,
 		case Opt_check_n:
 			opts->name_check = 'n';
 			break;
+		case Opt_usefree:
+			opts->usefree = 1;
+			break;
 		case Opt_nocase:
 			if (!is_vfat)
 				opts->nocase = 1;
@@ -1304,7 +1310,9 @@ int fat_fill_super(struct super_block *sb, void *data, int silent,
 			       le32_to_cpu(fsinfo->signature2),
 			       sbi->fsinfo_sector);
 		} else {
-			sbi->free_clusters = le32_to_cpu(fsinfo->free_clusters);
+			if (sbi->options.usefree)
+				sbi->free_clusters =
+					le32_to_cpu(fsinfo->free_clusters);
 			sbi->prev_free = le32_to_cpu(fsinfo->next_cluster);
 		}
 

include/linux/msdos_fs.h

@@ -205,7 +205,8 @@ struct fat_mount_options {
 		 numtail:1,       /* Does first alias have a numeric '~1' type tail? */
 		 atari:1,         /* Use Atari GEMDOS variation of MS-DOS fs */
 		 flush:1,	  /* write things quickly */
-		 nocase:1;	  /* Does this need case conversion? 0=need case conversion*/
+		 nocase:1,	  /* Does this need case conversion? 0=need case conversion*/
+		 usefree:1;	  /* Use free_clusters for FAT32 */
 };
 
 #define FAT_HASH_BITS	8