commit 3c0ed9c3a561ca744b2b76921bb8352ba2340669
author Stefan Brüns <stefan.bruens@rwth-aachen.de> Sun Sep 11 22:51:40 2016 +0200
committer Tom Rini <trini@konsulko.com> Fri Sep 23 08:55:56 2016 -0400
tree 96eaf61ad2db6e61e4b17944aa222c914cb07b49
parent ed76f912777066c788ae7e9cfb1d5e19e8c35274

fs/fat: Do not write unmodified fat entries to disk

The code caches 6 sectors of the FAT. On FAT traversal, the old contents
needs to be flushed to disk, but only if any FAT entries had been modified.
Explicitly flag the buffer on modification.

Currently, creating a new file traverses the whole FAT up to the first
free cluster and rewrites the on-disk blocks.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>

fs/fat/fat.c

diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index 826bd85..df9f2b5 100644
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -876,6 +876,7 @@
 	}
 
 	mydata->fatbufnum = -1;
+	mydata->fat_dirty = 0;
 	mydata->fatbuf = memalign(ARCH_DMA_MINALIGN, FATBUFSIZE);
 	if (mydata->fatbuf == NULL) {
 		debug("Error: allocating memory\n");

fs/fat/fat_write.c

diff --git a/fs/fat/fat_write.c b/fs/fat/fat_write.c
index 961ccd8..babe9c8 100644
--- a/fs/fat/fat_write.c
+++ b/fs/fat/fat_write.c
@@ -104,13 +104,19 @@
 /*
  * Write fat buffer into block device
  */
-static int flush_fat_buffer(fsdata *mydata)
+static int flush_dirty_fat_buffer(fsdata *mydata)
 {
 	int getsize = FATBUFBLOCKS;
 	__u32 fatlength = mydata->fatlength;
 	__u8 *bufptr = mydata->fatbuf;
 	__u32 startblock = mydata->fatbufnum * FATBUFBLOCKS;
 
+	debug("debug: evicting %d, dirty: %d\n", mydata->fatbufnum,
+	      (int)mydata->fat_dirty);
+
+	if ((!mydata->fat_dirty) || (mydata->fatbufnum == -1))
+		return 0;
+
 	startblock += mydata->fat_sect;
 
 	if (getsize > fatlength)
@@ -130,6 +136,7 @@
 			return -1;
 		}
 	}
+	mydata->fat_dirty = 0;
 
 	return 0;
 }
@@ -186,10 +193,8 @@
 		startblock += mydata->fat_sect;	/* Offset from start of disk */
 
 		/* Write back the fatbuf to the disk */
-		if (mydata->fatbufnum != -1) {
-			if (flush_fat_buffer(mydata) < 0)
-				return -1;
-		}
+		if (flush_dirty_fat_buffer(mydata) < 0)
+			return -1;
 
 		if (disk_read(startblock, getsize, bufptr) < 0) {
 			debug("Error reading FAT blocks\n");
@@ -494,10 +499,8 @@
 		if (getsize > fatlength)
 			getsize = fatlength;
 
-		if (mydata->fatbufnum != -1) {
-			if (flush_fat_buffer(mydata) < 0)
-				return -1;
-		}
+		if (flush_dirty_fat_buffer(mydata) < 0)
+			return -1;
 
 		if (disk_read(startblock, getsize, bufptr) < 0) {
 			debug("Error reading FAT blocks\n");
@@ -506,6 +509,9 @@
 		mydata->fatbufnum = bufnum;
 	}
 
+	/* Mark as dirty */
+	mydata->fat_dirty = 1;
+
 	/* Set the actual entry */
 	switch (mydata->fatsize) {
 	case 32:
@@ -645,7 +651,7 @@
 
 	dir_curclust = dir_newclust;
 
-	if (flush_fat_buffer(mydata) < 0)
+	if (flush_dirty_fat_buffer(mydata) < 0)
 		return;
 
 	memset(get_dentfromdir_block, 0x00,
@@ -675,7 +681,7 @@
 	}
 
 	/* Flush fat buffer */
-	if (flush_fat_buffer(mydata) < 0)
+	if (flush_dirty_fat_buffer(mydata) < 0)
 		return -1;
 
 	return 0;
@@ -1011,6 +1017,7 @@
 	}
 
 	mydata->fatbufnum = -1;
+	mydata->fat_dirty = 0;
 	mydata->fatbuf = memalign(ARCH_DMA_MINALIGN, FATBUFSIZE);
 	if (mydata->fatbuf == NULL) {
 		debug("Error: allocating memory\n");
@@ -1111,7 +1118,7 @@
 	debug("attempt to write 0x%llx bytes\n", *actwrite);
 
 	/* Flush fat buffer */
-	ret = flush_fat_buffer(mydata);
+	ret = flush_dirty_fat_buffer(mydata);
 	if (ret) {
 		printf("Error: flush fat buffer\n");
 		goto exit;