[PATCH] FAT: optimize FSINFO writeback

[steve]

Only write the FSINFO block back to disk when its contents change.
This optimization can be important when the underlying physical media
can wear out, i.e. Flash.

Signed-off-by: Steven J. Magnani <steve@xxxxxxxxxxxxxxx>
---
diff -uprN a/fs/fat/misc.c b/fs/fat/misc.c
--- a/fs/fat/misc.c	2009-06-29 11:12:40.000000000 -0500
+++ b/fs/fat/misc.c	2009-06-29 11:46:45.000000000 -0500
@@ -61,11 +61,25 @@ void fat_clusters_flush(struct super_blo
 		       le32_to_cpu(fsinfo->signature2),
 		       sbi->fsinfo_sector);
 	} else {
-		if (sbi->free_clusters != -1)
-			fsinfo->free_clusters = cpu_to_le32(sbi->free_clusters);
-		if (sbi->prev_free != -1)
-			fsinfo->next_cluster = cpu_to_le32(sbi->prev_free);
-		mark_buffer_dirty(bh);
+		char write_needed = 0;
+		__le32 le_value;
+
+		if (sbi->free_clusters != -1) {
+			le_value = cpu_to_le32(sbi->free_clusters);
+			if (fsinfo->free_clusters != le_value) {
+				fsinfo->free_clusters = le_value;
+				write_needed = 1;
+			}
+		}
+		if (sbi->prev_free != -1) {
+			le_value = cpu_to_le32(sbi->prev_free);
+			if (fsinfo->next_cluster != le_value) {
+				fsinfo->next_cluster = le_value;
+				write_needed = 1;
+			}
+		}
+		if (write_needed)
+			mark_buffer_dirty(bh);
 	}
 	brelse(bh);
 }

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