Re: Is there a "blackhole" /dev/null directory?
From: Bodo Eggert
Date: Thu Feb 14 2008 - 12:16:50 EST
rzryyvzy <rzryyvzy@xxxxxxxxxxxxx> wrote:
> /dev/null is often very useful, specially if programs force to save data in
> some file. But some programs like to creates different temporary file names,
> so /dev/null could no more work.
>
> What is with a "/dev/null"-directory?
> I mean a "blackhole pseudo directory" which eats every write to null.
>
> Here is how it could work:
> mount -t nulldir nulldir /dev/nulldir
>
> Now if a program does a create(2),
> it creates in the memory the file with its fd.
> Then if a program does a write(2) to the fd, it eats the writes and give out
> fakely it has written the number of bytes. When the program calls does a
> close(2) of the fd, then the complete inode is deleted in the memory.
>
> The directory should be permanently empty except for the inodes with open
> file descriptors. So only inode information would be temporary saved in this
> "nulldir tmpfs" directory.
>
> Is there already existing a possibility to create a null directory?
Please try the patch below. It will add an autounlink option to
tmpfs, which should automatically get rid of non-referenced files.
diff -X dontdiff -dpruN linux-2.6.24.pure/include/linux/shmem_fs.h
linux-2.6.24.autounlink/include/linux/shmem_fs.h
--- linux-2.6.24.pure/include/linux/shmem_fs.h 2006-11-29 22:57:37.000000000
+0100
+++ linux-2.6.24.autounlink/include/linux/shmem_fs.h 2008-02-14
15:35:01.000000000 +0100
@@ -30,11 +30,14 @@ struct shmem_sb_info {
unsigned long free_blocks; /* How many are left for allocation */
unsigned long max_inodes; /* How many inodes are allowed */
unsigned long free_inodes; /* How many are left for allocation */
- int policy; /* Default NUMA memory alloc policy */
- nodemask_t policy_nodes; /* nodemask for preferred and bind */
+ unsigned int flags;
+ int policy; /* Default NUMA memory alloc policy */
+ nodemask_t policy_nodes; /* nodemask for preferred and bind */
spinlock_t stat_lock;
};
+#define TMPFS_FL_AUTOREMOVE 1
+
static inline struct shmem_inode_info *SHMEM_I(struct inode *inode)
{
return container_of(inode, struct shmem_inode_info, vfs_inode);
diff -X dontdiff -dpruN linux-2.6.24.pure/mm/shmem.c
linux-2.6.24.autounlink/mm/shmem.c
--- linux-2.6.24.pure/mm/shmem.c 2008-01-25 15:09:39.000000000 +0100
+++ linux-2.6.24.autounlink/mm/shmem.c 2008-02-14 18:00:54.000000000 +0100
@@ -1747,31 +1747,41 @@ static int
shmem_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
{
struct inode *inode = shmem_get_inode(dir->i_sb, mode, dev);
+ struct shmem_sb_info *sbinfo = SHMEM_SB(dir->i_sb);
int error = -ENOSPC;
- if (inode) {
- error = security_inode_init_security(inode, dir, NULL, NULL,
- NULL);
- if (error) {
- if (error != -EOPNOTSUPP) {
- iput(inode);
- return error;
- }
- }
- error = shmem_acl_init(inode, dir);
- if (error) {
+ if (!inode)
+ return error;
+
+ error = security_inode_init_security(inode, dir, NULL, NULL,
+ NULL);
+ if (error) {
+ if (error != -EOPNOTSUPP) {
iput(inode);
return error;
}
- if (dir->i_mode & S_ISGID) {
- inode->i_gid = dir->i_gid;
- if (S_ISDIR(mode))
- inode->i_mode |= S_ISGID;
- }
- dir->i_size += BOGO_DIRENT_SIZE;
- dir->i_ctime = dir->i_mtime = CURRENT_TIME;
- d_instantiate(dentry, inode);
+ }
+ error = shmem_acl_init(inode, dir);
+ if (error) {
+ iput(inode);
+ return error;
+ }
+ if (dir->i_mode & S_ISGID) {
+ inode->i_gid = dir->i_gid;
+ if (S_ISDIR(mode))
+ inode->i_mode |= S_ISGID;
+ }
+
+ dir->i_size += BOGO_DIRENT_SIZE;
+ dir->i_ctime = dir->i_mtime = CURRENT_TIME;
+ d_instantiate(dentry, inode);
+ if ( S_ISDIR(mode)
+ || !(sbinfo->flags & TMPFS_FL_AUTOREMOVE))
+ {
dget(dentry); /* Extra count - pin the dentry in core */
+ } else {
+ dir->i_size -= BOGO_DIRENT_SIZE;
+ drop_nlink(inode);
}
return error;
}
@@ -1800,6 +1810,11 @@ static int shmem_link(struct dentry *old
struct inode *inode = old_dentry->d_inode;
struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
+ /* In auto-unlink mode, the newly created link would be unlinked
+ immediately. We don't need to do anything here. */
+ if (sbinfo->flags & TMPFS_FL_AUTOREMOVE)
+ return 0;
+
/*
* No ordinary (disk based) filesystem counts links as inodes;
* but each new link needs a new dentry, pinning lowmem, and
@@ -2095,6 +2110,7 @@ static const struct export_operations sh
static int shmem_parse_options(char *options, int *mode, uid_t *uid,
gid_t *gid, unsigned long *blocks, unsigned long *inodes,
+ unsigned int * flags,
int *policy, nodemask_t *policy_nodes)
{
char *this_char, *value, *rest;
@@ -2120,8 +2136,18 @@ static int shmem_parse_options(char *opt
continue;
if ((value = strchr(this_char,'=')) != NULL) {
*value++ = 0;
+
+ /* These options don't take arguments: */
+ } else if (!strcmp(this_char,"autounlink")) {
+ *flags |= TMPFS_FL_AUTOREMOVE;
+ continue;
+ } else if (!strcmp(this_char,"noautounlink")) {
+ *flags &= ~TMPFS_FL_AUTOREMOVE;
+ continue;
+
+ /* All other options need an argument */
} else {
- printk(KERN_ERR
+ printk(KERN_ERR
"tmpfs: No value for mount option '%s'\n",
this_char);
return 1;
@@ -2192,10 +2218,12 @@ static int shmem_remount_fs(struct super
nodemask_t policy_nodes = sbinfo->policy_nodes;
unsigned long blocks;
unsigned long inodes;
+ unsigned int sbflags;
int error = -EINVAL;
+ sbflags = sbinfo->flags;
if (shmem_parse_options(data, NULL, NULL, NULL, &max_blocks,
- &max_inodes, &policy, &policy_nodes))
+ &max_inodes, &sbflags, &policy, &policy_nodes))
return error;
spin_lock(&sbinfo->stat_lock);
@@ -2221,6 +2249,7 @@ static int shmem_remount_fs(struct super
sbinfo->free_blocks = max_blocks - blocks;
sbinfo->max_inodes = max_inodes;
sbinfo->free_inodes = max_inodes - inodes;
+ sbinfo->flags = sbflags;
sbinfo->policy = policy;
sbinfo->policy_nodes = policy_nodes;
out:
@@ -2247,6 +2276,7 @@ static int shmem_fill_super(struct super
struct shmem_sb_info *sbinfo;
unsigned long blocks = 0;
unsigned long inodes = 0;
+ unsigned int flags = 0;
int policy = MPOL_DEFAULT;
nodemask_t policy_nodes = node_states[N_HIGH_MEMORY];
@@ -2262,7 +2292,7 @@ static int shmem_fill_super(struct super
if (inodes > blocks)
inodes = blocks;
if (shmem_parse_options(data, &mode, &uid, &gid, &blocks,
- &inodes, &policy, &policy_nodes))
+ &inodes, &flags, &policy, &policy_nodes))
return -EINVAL;
}
sb->s_export_op = &shmem_export_ops;
@@ -2281,6 +2311,7 @@ static int shmem_fill_super(struct super
sbinfo->free_blocks = blocks;
sbinfo->max_inodes = inodes;
sbinfo->free_inodes = inodes;
+ sbinfo->flags = flags;
sbinfo->policy = policy;
sbinfo->policy_nodes = policy_nodes;
--
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