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

[Pavel Tikhomirov]

One more thing: inode can be reused, and I don't see where you handle 
it. E.g.: shmem_fh_to_dentry / shmem_match uses inode generation to 
detect it.

On 7/27/20 12:43 PM, Pavel Tikhomirov wrote:
> 
> 
> 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.