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cramfs: direct memory access support
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Small embedded systems typically execute the kernel code in place (XIP)
directly from flash to save on precious RAM usage. This patch adds to
cramfs the ability to consume filesystem data directly from flash as
well. Cramfs is particularly well suited to this feature as it is very
simple with low RAM usage, and with this feature it is possible to use
it with no block device support and consequently even lower RAM usage.

This patch was inspired by a similar patch from Shane Nay dated 17 years
ago that used to be very popular in embedded circles but never made it
into mainline. This is a cleaned-up implementation that uses far fewer
ifdef's and gets the actual memory location for the filesystem image
via MTD at run time. In the context of small IoT deployments, this
functionality has become relevant and useful again.

Signed-off-by: Nicolas Pitre <[email protected]>
Tested-by: Chris Brandt <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Al Viro <[email protected]>
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Nicolas Pitre authored and Al Viro committed Oct 15, 2017
1 parent 8a5776a commit 99c18ce
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Showing 2 changed files with 199 additions and 43 deletions.
30 changes: 29 additions & 1 deletion fs/cramfs/Kconfig
View file
Original file line number Diff line number Diff line change
@@ -1,6 +1,5 @@
config CRAMFS
tristate "Compressed ROM file system support (cramfs) (OBSOLETE)"
depends on BLOCK
select ZLIB_INFLATE
help
Saying Y here includes support for CramFs (Compressed ROM File
Expand All @@ -20,3 +19,32 @@ config CRAMFS
in terms of performance and features.

If unsure, say N.

config CRAMFS_BLOCKDEV
bool "Support CramFs image over a regular block device" if EXPERT
depends on CRAMFS && BLOCK
default y
help
This option allows the CramFs driver to load data from a regular
block device such a disk partition or a ramdisk.

config CRAMFS_MTD
bool "Support CramFs image directly mapped in physical memory"
depends on CRAMFS && MTD
default y if !CRAMFS_BLOCKDEV
help
This option allows the CramFs driver to load data directly from
a linear adressed memory range (usually non volatile memory
like flash) instead of going through the block device layer.
This saves some memory since no intermediate buffering is
necessary.

The location of the CramFs image is determined by a
MTD device capable of direct memory mapping e.g. from
the 'physmap' map driver or a resulting MTD partition.
For example, this would mount the cramfs image stored in
the MTD partition named "xip_fs" on the /mnt mountpoint:

mount -t cramfs mtd:xip_fs /mnt

If unsure, say N.
212 changes: 170 additions & 42 deletions fs/cramfs/inode.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,8 @@
#include <linux/init.h>
#include <linux/string.h>
#include <linux/blkdev.h>
#include <linux/mtd/mtd.h>
#include <linux/mtd/super.h>
#include <linux/slab.h>
#include <linux/vfs.h>
#include <linux/mutex.h>
Expand All @@ -36,6 +38,9 @@ struct cramfs_sb_info {
unsigned long blocks;
unsigned long files;
unsigned long flags;
void *linear_virt_addr;
resource_size_t linear_phys_addr;
size_t mtd_point_size;
};

static inline struct cramfs_sb_info *CRAMFS_SB(struct super_block *sb)
Expand Down Expand Up @@ -140,6 +145,9 @@ static struct inode *get_cramfs_inode(struct super_block *sb,
* BLKS_PER_BUF*PAGE_SIZE, so that the caller doesn't need to
* worry about end-of-buffer issues even when decompressing a full
* page cache.
*
* Note: This is all optimized away at compile time when
* CONFIG_CRAMFS_BLOCKDEV=n.
*/
#define READ_BUFFERS (2)
/* NEXT_BUFFER(): Loop over [0..(READ_BUFFERS-1)]. */
Expand All @@ -160,10 +168,10 @@ static struct super_block *buffer_dev[READ_BUFFERS];
static int next_buffer;

/*
* Returns a pointer to a buffer containing at least LEN bytes of
* filesystem starting at byte offset OFFSET into the filesystem.
* Populate our block cache and return a pointer to it.
*/
static void *cramfs_read(struct super_block *sb, unsigned int offset, unsigned int len)
static void *cramfs_blkdev_read(struct super_block *sb, unsigned int offset,
unsigned int len)
{
struct address_space *mapping = sb->s_bdev->bd_inode->i_mapping;
struct page *pages[BLKS_PER_BUF];
Expand Down Expand Up @@ -239,11 +247,49 @@ static void *cramfs_read(struct super_block *sb, unsigned int offset, unsigned i
return read_buffers[buffer] + offset;
}

/*
* Return a pointer to the linearly addressed cramfs image in memory.
*/
static void *cramfs_direct_read(struct super_block *sb, unsigned int offset,
unsigned int len)
{
struct cramfs_sb_info *sbi = CRAMFS_SB(sb);

if (!len)
return NULL;
if (len > sbi->size || offset > sbi->size - len)
return page_address(ZERO_PAGE(0));
return sbi->linear_virt_addr + offset;
}

/*
* Returns a pointer to a buffer containing at least LEN bytes of
* filesystem starting at byte offset OFFSET into the filesystem.
*/
static void *cramfs_read(struct super_block *sb, unsigned int offset,
unsigned int len)
{
struct cramfs_sb_info *sbi = CRAMFS_SB(sb);

if (IS_ENABLED(CONFIG_CRAMFS_MTD) && sbi->linear_virt_addr)
return cramfs_direct_read(sb, offset, len);
else if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV))
return cramfs_blkdev_read(sb, offset, len);
else
return NULL;
}

static void cramfs_kill_sb(struct super_block *sb)
{
struct cramfs_sb_info *sbi = CRAMFS_SB(sb);

kill_block_super(sb);
if (IS_ENABLED(CCONFIG_CRAMFS_MTD) && sb->s_mtd) {
if (sbi && sbi->mtd_point_size)
mtd_unpoint(sb->s_mtd, 0, sbi->mtd_point_size);
kill_mtd_super(sb);
} else if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV) && sb->s_bdev) {
kill_block_super(sb);
}
kfree(sbi);
}

Expand All @@ -254,45 +300,37 @@ static int cramfs_remount(struct super_block *sb, int *flags, char *data)
return 0;
}

static int cramfs_fill_super(struct super_block *sb, void *data, int silent)
static int cramfs_read_super(struct super_block *sb,
struct cramfs_super *super, int silent)
{
int i;
struct cramfs_super super;
struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
unsigned long root_offset;
struct cramfs_sb_info *sbi;
struct inode *root;

sb->s_flags |= MS_RDONLY;

sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
if (!sbi)
return -ENOMEM;
sb->s_fs_info = sbi;

/* Invalidate the read buffers on mount: think disk change.. */
mutex_lock(&read_mutex);
for (i = 0; i < READ_BUFFERS; i++)
buffer_blocknr[i] = -1;
/* We don't know the real size yet */
sbi->size = PAGE_SIZE;

/* Read the first block and get the superblock from it */
memcpy(&super, cramfs_read(sb, 0, sizeof(super)), sizeof(super));
mutex_lock(&read_mutex);
memcpy(super, cramfs_read(sb, 0, sizeof(*super)), sizeof(*super));
mutex_unlock(&read_mutex);

/* Do sanity checks on the superblock */
if (super.magic != CRAMFS_MAGIC) {
if (super->magic != CRAMFS_MAGIC) {
/* check for wrong endianness */
if (super.magic == CRAMFS_MAGIC_WEND) {
if (super->magic == CRAMFS_MAGIC_WEND) {
if (!silent)
pr_err("wrong endianness\n");
return -EINVAL;
}

/* check at 512 byte offset */
mutex_lock(&read_mutex);
memcpy(&super, cramfs_read(sb, 512, sizeof(super)), sizeof(super));
memcpy(super,
cramfs_read(sb, 512, sizeof(*super)),
sizeof(*super));
mutex_unlock(&read_mutex);
if (super.magic != CRAMFS_MAGIC) {
if (super.magic == CRAMFS_MAGIC_WEND && !silent)
if (super->magic != CRAMFS_MAGIC) {
if (super->magic == CRAMFS_MAGIC_WEND && !silent)
pr_err("wrong endianness\n");
else if (!silent)
pr_err("wrong magic\n");
Expand All @@ -301,44 +339,53 @@ static int cramfs_fill_super(struct super_block *sb, void *data, int silent)
}

/* get feature flags first */
if (super.flags & ~CRAMFS_SUPPORTED_FLAGS) {
if (super->flags & ~CRAMFS_SUPPORTED_FLAGS) {
pr_err("unsupported filesystem features\n");
return -EINVAL;
}

/* Check that the root inode is in a sane state */
if (!S_ISDIR(super.root.mode)) {
if (!S_ISDIR(super->root.mode)) {
pr_err("root is not a directory\n");
return -EINVAL;
}
/* correct strange, hard-coded permissions of mkcramfs */
super.root.mode |= (S_IRUSR | S_IXUSR | S_IRGRP | S_IXGRP | S_IROTH | S_IXOTH);
super->root.mode |= 0555;

root_offset = super.root.offset << 2;
if (super.flags & CRAMFS_FLAG_FSID_VERSION_2) {
sbi->size = super.size;
sbi->blocks = super.fsid.blocks;
sbi->files = super.fsid.files;
root_offset = super->root.offset << 2;
if (super->flags & CRAMFS_FLAG_FSID_VERSION_2) {
sbi->size = super->size;
sbi->blocks = super->fsid.blocks;
sbi->files = super->fsid.files;
} else {
sbi->size = 1<<28;
sbi->blocks = 0;
sbi->files = 0;
}
sbi->magic = super.magic;
sbi->flags = super.flags;
sbi->magic = super->magic;
sbi->flags = super->flags;
if (root_offset == 0)
pr_info("empty filesystem");
else if (!(super.flags & CRAMFS_FLAG_SHIFTED_ROOT_OFFSET) &&
else if (!(super->flags & CRAMFS_FLAG_SHIFTED_ROOT_OFFSET) &&
((root_offset != sizeof(struct cramfs_super)) &&
(root_offset != 512 + sizeof(struct cramfs_super))))
{
pr_err("bad root offset %lu\n", root_offset);
return -EINVAL;
}

return 0;
}

static int cramfs_finalize_super(struct super_block *sb,
struct cramfs_inode *cramfs_root)
{
struct inode *root;

/* Set it all up.. */
sb->s_flags |= MS_RDONLY;
sb->s_op = &cramfs_ops;
root = get_cramfs_inode(sb, &super.root, 0);
root = get_cramfs_inode(sb, cramfs_root, 0);
if (IS_ERR(root))
return PTR_ERR(root);
sb->s_root = d_make_root(root);
Expand All @@ -347,10 +394,79 @@ static int cramfs_fill_super(struct super_block *sb, void *data, int silent)
return 0;
}

static int cramfs_blkdev_fill_super(struct super_block *sb, void *data,
int silent)
{
struct cramfs_sb_info *sbi;
struct cramfs_super super;
int i, err;

sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
if (!sbi)
return -ENOMEM;
sb->s_fs_info = sbi;

/* Invalidate the read buffers on mount: think disk change.. */
for (i = 0; i < READ_BUFFERS; i++)
buffer_blocknr[i] = -1;

err = cramfs_read_super(sb, &super, silent);
if (err)
return err;
return cramfs_finalize_super(sb, &super.root);
}

static int cramfs_mtd_fill_super(struct super_block *sb, void *data,
int silent)
{
struct cramfs_sb_info *sbi;
struct cramfs_super super;
int err;

sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
if (!sbi)
return -ENOMEM;
sb->s_fs_info = sbi;

/* Map only one page for now. Will remap it when fs size is known. */
err = mtd_point(sb->s_mtd, 0, PAGE_SIZE, &sbi->mtd_point_size,
&sbi->linear_virt_addr, &sbi->linear_phys_addr);
if (err || sbi->mtd_point_size != PAGE_SIZE) {
pr_err("unable to get direct memory access to mtd:%s\n",
sb->s_mtd->name);
return err ? : -ENODATA;
}

pr_info("checking physical address %pap for linear cramfs image\n",
&sbi->linear_phys_addr);
err = cramfs_read_super(sb, &super, silent);
if (err)
return err;

/* Remap the whole filesystem now */
pr_info("linear cramfs image on mtd:%s appears to be %lu KB in size\n",
sb->s_mtd->name, sbi->size/1024);
mtd_unpoint(sb->s_mtd, 0, PAGE_SIZE);
err = mtd_point(sb->s_mtd, 0, sbi->size, &sbi->mtd_point_size,
&sbi->linear_virt_addr, &sbi->linear_phys_addr);
if (err || sbi->mtd_point_size != sbi->size) {
pr_err("unable to get direct memory access to mtd:%s\n",
sb->s_mtd->name);
return err ? : -ENODATA;
}

return cramfs_finalize_super(sb, &super.root);
}

static int cramfs_statfs(struct dentry *dentry, struct kstatfs *buf)
{
struct super_block *sb = dentry->d_sb;
u64 id = huge_encode_dev(sb->s_bdev->bd_dev);
u64 id = 0;

if (sb->s_bdev)
id = huge_encode_dev(sb->s_bdev->bd_dev);
else if (sb->s_dev)
id = huge_encode_dev(sb->s_dev);

buf->f_type = CRAMFS_MAGIC;
buf->f_bsize = PAGE_SIZE;
Expand Down Expand Up @@ -573,10 +689,22 @@ static const struct super_operations cramfs_ops = {
.statfs = cramfs_statfs,
};

static struct dentry *cramfs_mount(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data)
static struct dentry *cramfs_mount(struct file_system_type *fs_type, int flags,
const char *dev_name, void *data)
{
return mount_bdev(fs_type, flags, dev_name, data, cramfs_fill_super);
struct dentry *ret = ERR_PTR(-ENOPROTOOPT);

if (IS_ENABLED(CONFIG_CRAMFS_MTD)) {
ret = mount_mtd(fs_type, flags, dev_name, data,
cramfs_mtd_fill_super);
if (!IS_ERR(ret))
return ret;
}
if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV)) {
ret = mount_bdev(fs_type, flags, dev_name, data,
cramfs_blkdev_fill_super);
}
return ret;
}

static struct file_system_type cramfs_fs_type = {
Expand Down

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