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 <ptikhomi...@virtuozzo.com> Отправлено: 29 июля 2020 г. 13:46 Кому: Andrey Zhadchenko <andrey.zhadche...@virtuozzo.com>; devel@openvz.org <devel@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.zhadche...@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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