[Devel] [PATCH RH7 v2] cgroup: add export_operations to cgroup super block

[Pavel Tikhomirov]

On 7/24/20 6:50 PM, Andrey Zhadchenko wrote:
> criu uses fhandle from fdinfo to dump inotify objects. cgroup super block has
> no export operations, but .encode_fh and .fh_to_dentry are needed for
> inotify_fdinfo function and open_by_handle_at syscall in order to correctly
> open files located on cgroupfs by fhandle.
> Add hash table as a storage for inodes with exported fhandle.
> 
> https://jira.sw.ru/browse/PSBM-105889
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
> ---
>   kernel/cgroup.c | 112 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 111 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 9fdba79..a459c56 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -62,6 +62,7 @@
>   #include <linux/kthread.h>
>   #include <linux/ve.h>
>   #include <linux/stacktrace.h>
> +#include <linux/exportfs.h>
>   
>   #include <linux/atomic.h>
>   
> @@ -1390,9 +1391,117 @@ out:
>   }
>   #endif
>   
> +/*
> + * hashtable for inodes that have exported fhandles.
> + * When we export fhandle, we add it's inode into
> + * hashtable so we can find it fast
> + */
> +
> +#define CGROUP_INODE_HASH_BITS 10
> +static DEFINE_HASHTABLE(cgroup_inode_table, CGROUP_INODE_HASH_BITS);
> +static DEFINE_SPINLOCK(cgroup_inode_table_lock);
> +
> +struct cg_inode_hitem {
> +	struct inode *inode;
> +	struct hlist_node hlist;
> +};
> +
> +static inline unsigned long cgroup_inode_get_hash(unsigned int i_ino)
> +{
> +	return hash_32(i_ino, CGROUP_INODE_HASH_BITS);
> +}
> +
> +static struct cg_inode_hitem *cgroup_inode_hash_find(unsigned int i_ino)
> +{
> +	struct cg_inode_hitem *i;
> +	struct hlist_head *head = cgroup_inode_table
> +		+ cgroup_inode_get_hash(i_ino);
> +	struct cg_inode_hitem *found = 0;

NULL would look better than 0 for pointer.

> +
> +	spin_lock(&cgroup_inode_table_lock);
> +	hlist_for_each_entry(i, head, hlist) {
> +		if (i->inode->i_ino == i_ino) {
> +			found = i;
> +			break;
> +		}
> +	}
> +	spin_unlock(&cgroup_inode_table_lock);

The cg_inode_hitem can be freed from other thread just after you release 
the lock. Inode can be evicted and freed just after you release the 
lock. We should iget a reference on inode before releasing lock and 
return inode instead of cg_inode_hitem.

> +
> +	return found;
> +}
> +
> +static struct dentry *cgroup_fh_to_dentry(struct super_block *sb,
> +		struct fid *fid, int fh_len, int fh_type)
> +{
> +	struct cg_inode_hitem *item;
> +	struct dentry *dentry = ERR_PTR(-ENOENT);
> +
> +	if (fh_len < 1)
> +		return NULL;
> +
> +	item = cgroup_inode_hash_find(fid->raw[0]);
> +	if (item)
> +		dentry = d_find_alias(item->inode);

When you've got dentry you can put the inode reference back. See how 
it's done in shmem_fh_to_dentry.

> +
> +	return dentry;
> +}
> +
> +static int cgroup_encode_fh(struct inode *inode, __u32 *fh, int *len,
> +				struct inode *parent)
> +{
> +	struct hlist_head *head = cgroup_inode_table
> +		+ cgroup_inode_get_hash(inode->i_ino);
> +	struct cg_inode_hitem *item;
> +
> +	if (*len < 1) {
> +		*len = 1;
> +		return FILEID_INVALID;
> +	}
> +
> +	if (cgroup_inode_hash_find(inode->i_ino) == 0) {

Would look better:
	if (!cgroup_inode_hash_find(inode->i_ino)) {

> +		item = kmalloc(sizeof(struct cg_inode_hitem),
> +			GFP_KERNEL);
> +		/*
> +		 * encode_fh is expected to return 255 (FILEID_INVALID)
> +		 * in case of failure. We can't return ENOMEM, so
> +		 * return FILEID_INVALID at least
> +		 */
> +		if (!item)
> +			return FILEID_INVALID;
> +		item->inode = inode;
> +
> +		spin_lock(&cgroup_inode_table_lock);
> +		hlist_add_head(&item->hlist, head);
> +		spin_unlock(&cgroup_inode_table_lock);
> +	}
> +
> +	fh[0] = inode->i_ino;
> +	*len = 1;
> +	return 1;
> +}
> +
> +static const struct export_operations cgroup_export_ops = {
> +	.encode_fh      = cgroup_encode_fh,
> +	.fh_to_dentry	= cgroup_fh_to_dentry,
> +};
> +
> +static int cgroup_delete_inode(struct inode *inode)
> +{
> +	struct cg_inode_hitem *item = cgroup_inode_hash_find(inode->i_ino);

I don't like this. Lets only merge variable initialization and 
declaration if You don't expect any fail. And put empty line between 
declaration and everything else.

> +	if (item) {
> +		spin_lock(&cgroup_inode_table_lock);
> +		hlist_del(&item->hlist);
> +		spin_unlock(&cgroup_inode_table_lock);
> +
> +		kfree(item);
> +	}
> +
> +	return generic_delete_inode(inode);
> +}
> +
>   static const struct super_operations cgroup_ops = {
>   	.statfs = simple_statfs,
> -	.drop_inode = generic_delete_inode,
> +	.drop_inode = cgroup_delete_inode,
>   	.show_options = cgroup_show_options,
>   #ifdef CONFIG_VE
>   	.show_path = cgroup_show_path,
> @@ -1539,6 +1648,7 @@ static int cgroup_set_super(struct super_block *sb, void *data)
>   	sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
>   	sb->s_magic = CGROUP_SUPER_MAGIC;
>   	sb->s_op = &cgroup_ops;
> +	sb->s_export_op = &cgroup_export_ops;
>   
>   	return 0;
>   }
> 

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.