[PATCH and RFC] Compact time in cramfs

[PATCH and RFC] Compact time in cramfs

[Johan Adolfsson]

The following patch gives a cramfs filesystem a single timestamp
stored in the superblock. It uses the "future" field so no space
is wasted.
mkcramfs uses the newest mtime or ctime from the filesystem.

We is this feature good then?
For example: embedded systems with a webserver can now,
instead of simply generating a Last-Modified: header with
Jan 01 1970.. that the browser will use when caching stuff
provide an accurate timestamp that will change between
firmware releases making caching in the browser work better.

Patch is against 2.4.18 for everything except mkcramfs.c
which is a heavily hacked version although you should get the idea.
It has moved from the kernel anyway in 2.4.19-pre7.

Does it look ok?
Obvious improvements are to rename the future field in the superblock
to fstime and remove the fprintf.

/Johan



Index: linux/scripts/cramfs/mkcramfs.c
===================================================================
RCS file: /n/cvsroot/os/linux/scripts/cramfs/mkcramfs.c,v
retrieving revision 1.8
diff -u -p -r1.8 mkcramfs.c
--- linux/scripts/cramfs/mkcramfs.c	7 Sep 2001 11:31:28 -0000	1.8
+++ linux/scripts/cramfs/mkcramfs.c	23 Apr 2002 15:26:16 -0000
@@ -34,6 +34,7 @@
 #include <getopt.h>
 #include <linux/cramfs_fs.h>
 #include <zlib.h>
+#include <time.h>

 #define PAD_SIZE 512		/* only 0 and 512 supported by kernel */

@@ -71,6 +72,8 @@ static unsigned int blksize = DEFAULT_PA
 static long total_blocks = 0, total_nodes = 1; /* pre-count the root node */
 static int image_length = 0;

+static time_t newest_time = 0; /* Newest file in the filesystem */
+
 /*
  * If opt_holes is set, then mkcramfs can create explicit holes in the
  * data, which saves 26 bytes per hole (which is a lot smaller a
@@ -576,6 +579,13 @@ static unsigned int parse_directory(stru
 			warn_skip = 1;
 			continue;
 		}
+		/* Update the newest time */
+		if (st.st_mtime > newest_time) {
+			newest_time = st.st_mtime;
+		}
+		if (st.st_ctime > newest_time) {
+			newest_time = st.st_ctime;
+		}
 		entry = calloc(1, sizeof(struct entry));
 		if (!entry) {
 			perror(NULL);
@@ -708,7 +718,8 @@ static unsigned int write_superblock(str
 	if (opt_pad) {
 		offset += opt_pad;
 	}
-
+	fprintf(stderr, "Newest time: %lu = %s \n", (unsigned long)newest_time, ctime(&newest_time));
+	super->future = newest_time;
 	super->magic = CRAMFS_MAGIC;
 	super->flags = CRAMFS_FLAG_FSID_VERSION_2 | CRAMFS_FLAG_SORTED_DIRS;
 	if (opt_holes)
Index: linux/include/linux/cramfs_fs.h
===================================================================
RCS file: /n/cvsroot/os/linux/include/linux/cramfs_fs.h,v
retrieving revision 1.1.1.1
diff -u -p -r1.1.1.1 cramfs_fs.h
--- linux/include/linux/cramfs_fs.h	24 Jul 2001 12:41:48 -0000	1.1.1.1
+++ linux/include/linux/cramfs_fs.h	23 Apr 2002 15:26:24 -0000
@@ -55,7 +55,7 @@ struct cramfs_super {
 	u32 magic;		/* 0x28cd3d45 - random number */
 	u32 size;		/* length in bytes */
 	u32 flags;		/* 0 */
-	u32 future;		/* 0 */
+	u32 future;		/* Newest ctime or mtime for the filesystem */
 	u8 signature[16];	/* "Compressed ROMFS" */
 	struct cramfs_info fsid;	/* unique filesystem info */
 	u8 name[16];		/* user-defined name */
Index: linux/include/linux/cramfs_fs_sb.h
===================================================================
RCS file: /n/cvsroot/os/linux/include/linux/cramfs_fs_sb.h,v
retrieving revision 1.1.1.1
diff -u -p -r1.1.1.1 cramfs_fs_sb.h
--- linux/include/linux/cramfs_fs_sb.h	24 Jul 2001 12:41:48 -0000	1.1.1.1
+++ linux/include/linux/cramfs_fs_sb.h	23 Apr 2002 15:26:29 -0000
@@ -5,11 +5,12 @@
  * cramfs super-block data in memory
  */
 struct cramfs_sb_info {
-			unsigned long magic;
-			unsigned long size;
-			unsigned long blocks;
-			unsigned long files;
-			unsigned long flags;
+	unsigned long magic;
+	unsigned long size;
+	unsigned long blocks;
+	unsigned long files;
+	unsigned long flags;
+	unsigned long fstime; /* From the future field, max(mtime, ctime) */
 };

 #endif
Index: linux/fs/cramfs/inode.c
===================================================================
RCS file: /n/cvsroot/os/linux/fs/cramfs/inode.c,v
retrieving revision 1.16
diff -u -p -r1.16 inode.c
--- linux/fs/cramfs/inode.c	12 Apr 2002 11:38:08 -0000	1.16
+++ linux/fs/cramfs/inode.c	23 Apr 2002 15:26:35 -0000
@@ -28,6 +28,7 @@
 #define CRAMFS_SB_BLOCKS u.cramfs_sb.blocks
 #define CRAMFS_SB_FILES u.cramfs_sb.files
 #define CRAMFS_SB_FLAGS u.cramfs_sb.flags
+#define CRAMFS_SB_FSTIME u.cramfs_sb.fstime

 #define CRAMFS_AS_IMAGE

@@ -56,6 +57,8 @@ static struct inode *get_cramfs_inode(st
 		inode->i_blksize = PAGE_CACHE_SIZE;
 		inode->i_gid = cramfs_inode->gid;
 		inode->i_ino = CRAMINO(cramfs_inode);
+		inode->i_mtime = sb->CRAMFS_SB_FSTIME;
+		inode->i_ctime = sb->CRAMFS_SB_FSTIME;
 		/* inode->i_nlink is left 1 - arguably wrong for directories,
 		   but it's the best we can do without reading the directory
 	           contents.  1 yields the right result in GNU find, even
@@ -267,6 +270,7 @@ static struct super_block * cramfs_read_
 	}
 	sb->CRAMFS_SB_MAGIC=super.magic;
 	sb->CRAMFS_SB_FLAGS=super.flags;
+	sb->CRAMFS_SB_FSTIME=super.future;
 	if (root_offset == 0)
 		printk(KERN_INFO "cramfs: empty filesystem");
 	else if (!(super.flags & CRAMFS_FLAG_SHIFTED_ROOT_OFFSET) &&

Re: [PATCH and RFC] Compact time in cramfs

[johan.adolfsson]

Daniel Quinlan writes:


> Johan Adolfsson writes:
>
> > The following patch gives a cramfs filesystem a single timestamp
> > stored in the superblock. It uses the "future" field so no space is
> > wasted.  mkcramfs uses the newest mtime or ctime from the
> > filesystem.
>
> Agreed, it seems like a good idea to have a filesystem timestamp.
>
> The future field should probably be saved for use as the offset of a
> "secondary superblock" (or something like that) rather than squandered
> for one field.  At least, I think that was the original intent for the
> field and it has always been my plan.

Ok, I wondered what the intent really was.

> Maybe it would be easiest to overload the super.fsid.edition field?

Makes sense.

> Then you could have an option (probably a backward-compatible flag in
> the superblock, but could be compile-time or mount-time) that indicates
> that the edition number is a timestamp.

Using a flag should be enough I think:
#define CRAMFS_FLAG_FSID_EDITION_IS_TIMESTAMP  0x00000004 /* edition field
used as timestamp */

And mkcramfs should perhaps have support for
-e timestamp
(the string timestamp) which would set the edition field to the timestamp
according to the files in the image and set the timestamp flag.
Perhaps another option to set the timestamp flag but use whatever number
is supplied in the -e option might be useful for some, although I personally
don't think I need it. E.g. -f FSID_EDITION_IS_TIMESTAMP


Another mkcramfs feature we have at Axis is the support for metafiles with
file info used when creating the image. That allows us to create device
nodes, change userid etc. without being root which is good when building
for embedded devices.
It was first posted in October last year but without any feedback:
http://www.ussg.iu.edu/hypermail/linux/kernel/0110.0/0098.html
What do you think about that?

> - Dan

/Johan

Re: [PATCH and RFC] Compact time in cramfs

[Daniel Quinlan]

Johan Adolfsson writes:

> The following patch gives a cramfs filesystem a single timestamp
> stored in the superblock. It uses the "future" field so no space is
> wasted.  mkcramfs uses the newest mtime or ctime from the
> filesystem.

Agreed, it seems like a good idea to have a filesystem timestamp.

The future field should probably be saved for use as the offset of a
"secondary superblock" (or something like that) rather than squandered
for one field.  At least, I think that was the original intent for the
field and it has always been my plan.

Maybe it would be easiest to overload the super.fsid.edition field?
Then you could have an option (probably a backward-compatible flag in
the superblock, but could be compile-time or mount-time) that indicates
that the edition number is a timestamp.

- Dan