how are inode numbers generated in linux tmpfs? - linux

Question

It seems to me that tmpfs is not re-using inode numbers, but instead creates a new inode number via a +1 sequence everytime it needs a free inode.
Do you know how this is implemented / can you pin-point me to some source code where i could check the algorithm that is used in tmpfs ?
I need to understand this in order to bypass a limitation in a caching system that uses the inode number as its cache key (hence leading to rare, but occuring collisions when inodes are re-used too often). tmpfs could save my day if I can prove that it keeps creating unique inode numbers.
Thank you for your help,
Jerome Wagner

Answer 1

I won't directly answer your question, so I apologize in advance for that.
The tmpfs idea is good, but I wouldn't have my program depend on a more or less obscure implementation detail for generating keys. Why don't you try another method, such as combining the inode number with some other information? Maybe modification date: it's impossible two files get the same inode number AND modification date at the time of key-generation, unless system date changes.
Cheers!

Answer 2

The bulk of the tmpfs code is in mm/shmem.c. New inodes are created by
static struct inode *shmem_get_inode(struct super_block *sb, const struct inode *dir,
int mode, dev_t dev, unsigned long flags)
but it delegates almost everything to the generic filesystem code.
In particular, the field i_ino is filled in in fs/inode.c:
/**
* new_inode - obtain an inode
* #sb: superblock
*
* Allocates a new inode for given superblock. The default gfp_mask
* for allocations related to inode->i_mapping is GFP_HIGHUSER_MOVABLE.
* If HIGHMEM pages are unsuitable or it is known that pages allocated
* for the page cache are not reclaimable or migratable,
* mapping_set_gfp_mask() must be called with suitable flags on the
* newly created inode's mapping
*
*/
struct inode *new_inode(struct super_block *sb)
{
/*
* On a 32bit, non LFS stat() call, glibc will generate an EOVERFLOW
* error if st_ino won't fit in target struct field. Use 32bit counter
* here to attempt to avoid that.
*/
static unsigned int last_ino;
struct inode *inode;
spin_lock_prefetch(&inode_lock);
inode = alloc_inode(sb);
if (inode) {
spin_lock(&inode_lock);
__inode_add_to_lists(sb, NULL, inode);
inode->i_ino = ++last_ino;
inode->i_state = 0;
spin_unlock(&inode_lock);
}
return inode;
}
And it does indeed just use an incrementing counter (last_ino).
Most other filesystems use information from the on-disk files to later override the i_ino field.
Note that it's perfectly possible for this to wrap all the way around. The kernel also has a "generation" field that gets filled in various ways. mm/shmem.c uses the current time.