[Devel] [PATCH RH7 v3] cgroup: add export_operations to cgroup super block
Andrey Zhadchenko
andrey.zhadchenko at virtuozzo.com
Wed Jul 29 14:07:37 MSK 2020
I did look into that matter, and here is my thoughts.
So far if inode ever reach i_count == 0, iput will call iput_final,
which will call drop function sb->op->drop_inode, set I_FREEING status
and then call evict. I don't really think any sane code
will ever increment i_count back.
________________________________
От: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Отправлено: 29 июля 2020 г. 13:46
Кому: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>; devel at openvz.org <devel at openvz.org>
Тема: Re: [PATCH RH7 v3] cgroup: add export_operations to cgroup super block
Except for two small nits it looks good.
Note: Because we use atomic_inc_not_zero(i_count) and the fact that iput
can increment i_count back, we can _potentially_ see situation that
open_by_handle_at first fails to open and next is able to open the same
handle. But I don't think it's a big deal for us.
On 7/28/20 2:59 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.
>
> v3: use inode->i_gen to protect from i_ino reusage. increase fhandle size to
> 2 * u32.
> Add an option to take reference of inode in cgroup_find_inode, so no one can
> delete recently found inode.
>
> https://jira.sw.ru/browse/PSBM-105889
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
> ---
> kernel/cgroup.c | 137 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 136 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 9fdba79..267a5a4 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -62,6 +62,8 @@
> #include <linux/kthread.h>
> #include <linux/ve.h>
> #include <linux/stacktrace.h>
> +#include <linux/exportfs.h>
> +#include <linux/time.h>
>
> #include <linux/atomic.h>
>
> @@ -765,6 +767,7 @@ static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
>
> if (inode) {
> inode->i_ino = get_next_ino();
> + inode->i_generation = get_seconds();
We likely want to use prandom_u32() instead, see mainstream linux commit
46c9a946d766 ("shmem: use monotonic time for i_generation"), they've
change it for shmem to overcome overflow.
> inode->i_mode = mode;
> inode->i_uid = current_fsuid();
> inode->i_gid = current_fsgid();
> @@ -1390,9 +1393,140 @@ 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_find_inode(unsigned long fhandle[2],
> + char take_ref)
> +{
> + struct cg_inode_hitem *i;
> + struct hlist_head *head = cgroup_inode_table
> + + cgroup_inode_get_hash(fhandle[1]);
> + struct cg_inode_hitem *found = NULL;
> +
> + spin_lock(&cgroup_inode_table_lock);
> + hlist_for_each_entry(i, head, hlist) {
> + if (i->inode->i_generation == fhandle[0] &&
> + i->inode->i_ino == fhandle[1]) {
> + /*
> + * If we need to increase refcount, we should be aware
> + * of possible deadlock. Another thread may have
> + * started deleting this inode: iput->iput_final->
> + * ->cgroup_delete_inode->cgroup_find_inode. if we
> + * just call igrab, it will try to take i_lock and
> + * this will result in deadlock, because deleting
> + * thread has already taken it and waits on
> + * cgroup_inode_table_lock to find inode in hashtable.
> + * If i_count is zero, someone is deleting it -> skip.
> + */
> + if (take_ref)
> + if (!atomic_inc_not_zero(&i->inode->i_count))
> + continue;
Should be break instead of continue? There can't be another entry in
hash with same i_ino and i_generation.
> + found = i;
> + break;
> + }
> + }
> + spin_unlock(&cgroup_inode_table_lock);
> +
> + 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);
> + unsigned long fhandle[2] = {fid->raw[0], fid->raw[1]};
> +
> + if (fh_len < 2)
> + return NULL;
> +
> + item = cgroup_find_inode(fhandle, 1);
> + if (item) {
> + dentry = d_find_alias(item->inode);
> + iput(item->inode);
> + }
> + return dentry;
> +}
> +
> +static int cgroup_encode_fh(struct inode *inode, __u32 *fh, int *len,
> + struct inode *parent)
> +{
> + struct cg_inode_hitem *item;
> + struct hlist_head *head = cgroup_inode_table
> + + cgroup_inode_get_hash(inode->i_ino);
> + unsigned long fhandle[2] = {inode->i_generation, inode->i_ino};
> +
> + if (*len < 2) {
> + *len = 2;
> + return FILEID_INVALID;
> + }
> +
> + if (!cgroup_find_inode(fhandle, 0)) {
> + 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] = fhandle[0];
> + fh[1] = fhandle[1];
> + *len = 2;
> + 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 = NULL;
> + unsigned long fhandle[2] = {inode->i_generation, inode->i_ino};
> +
> + item = cgroup_find_inode(fhandle, 0);
> + 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 +1673,7 @@ 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.
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