这部分主要对linux虚拟文件系统内核初始化部分做些补充。
关于shrinker,inode和dentry cache初始化阶段都需要注册自己的shrinker,用于缩减cache。两个操作原理类似。
shrinker数据结构介绍
[cpp]
- /*
- * A callback you can register to apply pressure to ageable caches.
- *
- * ‘shrink’ is passed a count ‘nr_to_scan’ and a ‘gfpmask’. It should
- * look through the least-recently-used ‘nr_to_scan’ entries and
- * attempt to free them up. It should return the number of objects
- * which remain in the cache. If it returns -1, it means it cannot do
- * any scanning at this time (eg. there is a risk of deadlock).
- *
- * The ‘gfpmask’ refers to the allocation we are currently trying to
- * fulfil.
- *
- * Note that ‘shrink’ will be passed nr_to_scan == 0 when the VM is
- * querying the cache size, so a fastpath for that case is appropriate.
- */
- struct shrinker {
- int (*shrink)(int nr_to_scan, gfp_t gfp_mask);
- int seeks; /* seeks to recreate an obj */
- /* These are for internal use */
- struct list_head list;
- long nr; /* objs pending delete */
- };
1,注册inode cache shrinker
Start_kernel()->vfs_caches_init()->dcache_init()->register_shrinker(&dcache_shrinker);
[cpp]
- /*
- * Add a shrinker callback to be called from the vm
- */
- void register_shrinker(struct shrinker *shrinker)
- {
- shrinker->nr = 0;
- down_write(&shrinker_rwsem);
- list_add_tail(&shrinker->list, &shrinker_list);
- up_write(&shrinker_rwsem);
- }
其中相关的函数在这里定义。
[cpp]
- static struct shrinker dcache_shrinker = {
- .shrink = shrink_dcache_memory,
- .seeks = DEFAULT_SEEKS,
- };
[cpp]
- /*
- * Scan `nr’ dentries and return the number which remain.
- *
- * We need to avoid reentering the filesystem if the caller is performing a
- * GFP_NOFS allocation attempt. One example deadlock is:
- *
- * ext2_new_block->getblk->GFP->shrink_dcache_memory->prune_dcache->
- * prune_one_dentry->dput->dentry_iput->iput->inode->i_sb->s_op->put_inode->
- * ext2_discard_prealloc->ext2_free_blocks->lock_super->DEADLOCK.
- *
- * In this case we return -1 to tell the caller that we baled.
- */
- static int shrink_dcache_memory(int nr, gfp_t gfp_mask)
- {
- if (nr) {
- if (!(gfp_mask & __GFP_FS))
- return -1;
- prune_dcache(nr);/*缩减指定大小的cache*/
- }
- return (dentry_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
- }
[cpp]
- /**
- * prune_dcache – shrink the dcache
- * @count: number of entries to try to free
- *
- * Shrink the dcache. This is done when we need more memory, or simply when we
- * need to unmount something (at which point we need to unuse all dentries).
- *
- * This function may fail to free any resources if all the dentries are in use.
- */
- /*缩减dcache,count为释放的数量*/
- static void prune_dcache(int count)
- {
- struct super_block *sb;
- int w_count;
- int unused = dentry_stat.nr_unused;
- int prune_ratio;
- int pruned;
- if (unused == 0 || count == 0)
- return;
- spin_lock(&dcache_lock);
- restart:
- if (count >= unused)
- prune_ratio = 1;/*释放率*/
- else
- prune_ratio = unused / count;
- spin_lock(&sb_lock);
- list_for_each_entry(sb, &super_blocks, s_list) {
- if (sb->s_nr_dentry_unused == 0)
- continue;
- sb->s_count++;
- /* Now, we reclaim unused dentrins with fairness.
- * We reclaim them same percentage from each superblock.
- * We calculate number of dentries to scan on this sb
- * as follows, but the implementation is arranged to avoid
- * overflows:
- * number of dentries to scan on this sb =
- * count * (number of dentries on this sb /
- * number of dentries in the machine)
- */
- spin_unlock(&sb_lock);
- /*重新利用释放率计算释放量*/
- if (prune_ratio != 1)
- w_count = (sb->s_nr_dentry_unused / prune_ratio) + 1;
- else
- w_count = sb->s_nr_dentry_unused;
- pruned = w_count;
- /*
- * We need to be sure this filesystem isn’t being unmounted,
- * otherwise we could race with generic_shutdown_super(), and
- * end up holding a reference to an inode while the filesystem
- * is unmounted. So we try to get s_umount, and make sure
- * s_root isn’t NULL.
- */
- if (down_read_trylock(&sb->s_umount)) {
- if ((sb->s_root != NULL) &&
- (!list_empty(&sb->s_dentry_lru))) {
- spin_unlock(&dcache_lock);
- /*实际释放工作*/
- __shrink_dcache_sb(sb, &w_count,
- DCACHE_REFERENCED);
- pruned -= w_count;
- spin_lock(&dcache_lock);
- }
- up_read(&sb->s_umount);
- }
- spin_lock(&sb_lock);
- count -= pruned;
- /*
- * restart only when sb is no longer on the list and
- * we have more work to do.
- */
- if (__put_super_and_need_restart(sb) && count > 0) {
- spin_unlock(&sb_lock);
- goto restart;
- }
- }
- spin_unlock(&sb_lock);
- spin_unlock(&dcache_lock);
- }
[cpp]
- /*
- * Shrink the dentry LRU on a given superblock.
- * @sb : superblock to shrink dentry LRU.
- * @count: If count is NULL, we prune all dentries on superblock.
- * @flags: If flags is non-zero, we need to do special processing based on
- * which flags are set. This means we don’t need to maintain multiple
- * similar copies of this loop.
- */
- static void __shrink_dcache_sb(struct super_block *sb, int *count, int flags)
- {
- LIST_HEAD(referenced);
- LIST_HEAD(tmp);
- struct dentry *dentry;
- int cnt = 0;
- BUG_ON(!sb);
- BUG_ON((flags & DCACHE_REFERENCED) && count == NULL);
- spin_lock(&dcache_lock);
- if (count != NULL)
- /* called from prune_dcache() and shrink_dcache_parent() */
- cnt = *count;/*在下面用到*/
- restart:
- if (count == NULL)
- list_splice_init(&sb->s_dentry_lru, &tmp);
- else {
- while (!list_empty(&sb->s_dentry_lru)) {
- dentry = list_entry(sb->s_dentry_lru.prev,
- struct dentry, d_lru);
- BUG_ON(dentry->d_sb != sb);
- spin_lock(&dentry->d_lock);
- /*
- * If we are honouring the DCACHE_REFERENCED flag and
- * the dentry has this flag set, don’t free it. Clear
- * the flag and put it back on the LRU.
- */
- /*清flag对应位,将链表元素放LRU尾部*/
- if ((flags & DCACHE_REFERENCED)
- && (dentry->d_flags & DCACHE_REFERENCED)) {
- dentry->d_flags &= ~DCACHE_REFERENCED;
- list_move(&dentry->d_lru, &referenced);
- spin_unlock(&dentry->d_lock);
- } else {
- /*从d_lru链表中删除,加到tmp链表中*/
- list_move_tail(&dentry->d_lru, &tmp);
- spin_unlock(&dentry->d_lock);
- cnt–;/*数量减一*/
- if (!cnt)/*减到0跳出循环*/
- break;
- }
- cond_resched_lock(&dcache_lock);
- }
- }
- /*对tmp中的每个元素,其中tmp中的元素为上面移过来的*/
- while (!list_empty(&tmp)) {
- dentry = list_entry(tmp.prev, struct dentry, d_lru);
- /*从tmp中删除相关链表并做重新初始化和数据统计*/
- dentry_lru_del_init(dentry);
- spin_lock(&dentry->d_lock);
- /*
- * We found an inuse dentry which was not removed from
- * the LRU because of laziness during lookup. Do not free
- * it – just keep it off the LRU list.
- */
- if (atomic_read(&dentry->d_count)) {
- spin_unlock(&dentry->d_lock);
- continue;
- }/*释放dentry和其父dentry*/
- prune_one_dentry(dentry);
- /* dentry->d_lock was dropped in prune_one_dentry() */
- cond_resched_lock(&dcache_lock);
- }
- if (count == NULL && !list_empty(&sb->s_dentry_lru))
- goto restart;
- if (count != NULL)
- *count = cnt;
- if (!list_empty(&referenced))
- list_splice(&referenced, &sb->s_dentry_lru);
- spin_unlock(&dcache_lock);
- }
[cpp]
- static void dentry_lru_del_init(struct dentry *dentry)
- {
- if (likely(!list_empty(&dentry->d_lru))) {
- list_del_init(&dentry->d_lru);/*从链表中删除并初始化dentry->d_lru*/
- dentry->d_sb->s_nr_dentry_unused–;/*未用数减一*/
- dentry_stat.nr_unused–;/*更新统计数据*/
- }
- }
[html]
- /*
- * Throw away a dentry – free the inode, dput the parent. This requires that
- * the LRU list has already been removed.
- *
- * Try to prune ancestors as well. This is necessary to prevent
- * quadratic behavior of shrink_dcache_parent(), but is also expected
- * to be beneficial in reducing dentry cache fragmentation.
- */
- static void prune_one_dentry(struct dentry * dentry)
- __releases(dentry->d_lock)
- __releases(dcache_lock)
- __acquires(dcache_lock)
- {
- __d_drop(dentry);
- dentry = d_kill(dentry);/*释放dentry*/
- /*
- * Prune ancestors. Locking is simpler than in dput(),
- * because dcache_lock needs to be taken anyway.
- */
- spin_lock(&dcache_lock);
- while (dentry) {
- if (!atomic_dec_and_lock(&dentry->d_count, &dentry->d_lock))
- return;
- if (dentry->d_op && dentry->d_op->d_delete)
- dentry->d_op->d_delete(dentry);
- dentry_lru_del_init(dentry);
- __d_drop(dentry);
- dentry = d_kill(dentry);
- spin_lock(&dcache_lock);
- }
- }
[cpp]
- /**
- * d_kill – kill dentry and return parent
- * @dentry: dentry to kill
- *
- * The dentry must already be unhashed and removed from the LRU.
- *
- * If this is the root of the dentry tree, return NULL.
- */
- static struct dentry *d_kill(struct dentry *dentry)
- __releases(dentry->d_lock)
- __releases(dcache_lock)
- {
- struct dentry *parent;
- list_del(&dentry->d_u.d_child);/*删除子目录*/
- dentry_stat.nr_dentry–;/*更新统计数据*/ /* For d_free, below */
- /*drops the locks, at that point nobody can reach this dentry */
- dentry_iput(dentry);/*”释放”inode*/
- if (IS_ROOT(dentry))
- parent = NULL;
- else
- parent = dentry->d_parent;
- d_free(dentry);/*释放dentry*/
- return parent;
- }
[cpp]
- /*
- * Release the dentry’s inode, using the filesystem
- * d_iput() operation if defined.
- */
- /*释放inode*/
- static void dentry_iput(struct dentry * dentry)
- __releases(dentry->d_lock)
- __releases(dcache_lock)
- {
- struct inode *inode = dentry->d_inode;
- if (inode) {
- dentry->d_inode = NULL;
- list_del_init(&dentry->d_alias);/*从同一索引节点目录链表中删除*/
- spin_unlock(&dentry->d_lock);
- spin_unlock(&dcache_lock);
- if (!inode->i_nlink)/*如果inode没有硬链接*/
- fsnotify_inoderemove(inode);
- if (dentry->d_op && dentry->d_op->d_iput)
- dentry->d_op->d_iput(dentry, inode);
- else
- iput(inode);/*释放inode*/
- } else {
- spin_unlock(&dentry->d_lock);
- spin_unlock(&dcache_lock);
- }
- }
2.注册inode cache shrinker
Start_kernel()->vfs_caches_init()->inode_init()->register_shrinker(&icache_shrinker);
其中参数为下面定义
[cpp]
- static struct shrinker icache_shrinker = {
- .shrink = shrink_icache_memory,
- .seeks = DEFAULT_SEEKS,
- };
[cpp]
- static int shrink_icache_memory(int nr, gfp_t gfp_mask)
- {
- if (nr) {
- /*
- * Nasty deadlock avoidance. We may hold various FS locks,
- * and we don’t want to recurse into the FS that called us
- * in clear_inode() and friends..
- */
- if (!(gfp_mask & __GFP_FS))
- return -1;
- prune_icache(nr);
- }
- return (inodes_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
- }
[cpp]
- /*
- * Scan `goal’ inodes on the unused list for freeable ones. They are moved to
- * a temporary list and then are freed outside inode_lock by dispose_list().
- *
- * Any inodes which are pinned purely because of attached pagecache have their
- * pagecache removed. We expect the final iput() on that inode to add it to
- * the front of the inode_unused list. So look for it there and if the
- * inode is still freeable, proceed. The right inode is found 99.9% of the
- * time in testing on a 4-way.
- *
- * If the inode has metadata buffers attached to mapping->private_list then
- * try to remove them.
- */
- static void prune_icache(int nr_to_scan)
- {
- LIST_HEAD(freeable);/*初始化freeable,在下面需要用到,作为临时存放可被释放的inode*/
- int nr_pruned = 0;
- int nr_scanned;
- unsigned long reap = 0;
- down_read(&iprune_sem);
- spin_lock(&inode_lock);
- for (nr_scanned = 0; nr_scanned < nr_to_scan; nr_scanned++) {
- struct inode *inode;
- if (list_empty(&inode_unused))
- break;
- inode = list_entry(inode_unused.prev, struct inode, i_list);
- if (inode->i_state || atomic_read(&inode->i_count)) {
- /*将ionde从inode_unused链表中删除,加入inode_unused链表头*/
- list_move(&inode->i_list, &inode_unused);
- continue;
- }
- if (inode_has_buffers(inode) || inode->i_data.nrpages) {
- __iget(inode);/*移动到使用链表*/
- spin_unlock(&inode_lock);
- if (remove_inode_buffers(inode))/*从buffer链表中删除所有buffer*/
- reap += invalidate_mapping_pages(&inode->i_data,
- 0, -1);
- iput(inode);
- spin_lock(&inode_lock);
- if (inode != list_entry(inode_unused.next,
- struct inode, i_list))
- continue; /* wrong inode or list_empty */
- if (!can_unuse(inode))
- continue;
- }
- /*移动到freeable链表*/
- list_move(&inode->i_list, &freeable);
- WARN_ON(inode->i_state & I_NEW);
- inode->i_state |= I_FREEING;
- nr_pruned++;/*统计移动到freeable链表的元素个数*/
- }
- inodes_stat.nr_unused -= nr_pruned;/*更新统计数据*/
- if (current_is_kswapd())
- __count_vm_events(KSWAPD_INODESTEAL, reap);
- else
- __count_vm_events(PGINODESTEAL, reap);
- spin_unlock(&inode_lock);
- dispose_list(&freeable);/*将freeable链表中的数据处理掉*/
- up_read(&iprune_sem);
- }
3,注册文件描述符表释放函数
[cpp]
- /*文件描述符表*/
- struct fdtable {
- unsigned int max_fds;/*进程能够处理的最大file结构*/
- struct file ** fd;/*所有打开文件信息*//* current fd array */
- fd_set *close_on_exec;/*exec系统调用被关闭的所有文件集合*/
- fd_set *open_fds;/*当前打开的所有文件集合*/
- struct rcu_head rcu;
- struct fdtable *next;
- };
Start_kernel()->vfs_caches_init()->files_init()->files_defer_init()->fdtable_defer_list_init()->INIT_WORK(&fddef->wq, free_fdtable_work);
[cpp]
- static void free_fdtable_work(struct work_struct *work)
- {
- struct fdtable_defer *f =
- container_of(work, struct fdtable_defer, wq);
- struct fdtable *fdt;
- spin_lock_bh(&f->lock);
- fdt = f->next;
- f->next = NULL;
- spin_unlock_bh(&f->lock);
- while(fdt) {/*释放工作*/
- struct fdtable *next = fdt->next;
- vfree(fdt->fd);
- free_fdset(fdt);
- kfree(fdt);
- fdt = next;
- }
- }
4.sysfs文件系统初始化
Start_kernel()->vfs_caches_init()->mnt_init()->sysfs_init()
[cpp]
- int __init sysfs_init(void)
- {
- int err = -ENOMEM;
- sysfs_dir_cachep = kmem_cache_create(“sysfs_dir_cache”,
- sizeof(struct sysfs_dirent),
- 0, 0, NULL);
- if (!sysfs_dir_cachep)
- goto out;
- /*初始化sysfs的backing_dev_info结构*/
- err = sysfs_inode_init();
- if (err)
- goto out_err;
- /*注册文件系统*/
- err = register_filesystem(&sysfs_fs_type);
- if (!err) {
- /*创建sysfs mount*/
- sysfs_mount = kern_mount(&sysfs_fs_type);
- if (IS_ERR(sysfs_mount)) {
- printk(KERN_ERR “sysfs: could not mount!\n”);
- err = PTR_ERR(sysfs_mount);
- sysfs_mount = NULL;
- unregister_filesystem(&sysfs_fs_type);
- goto out_err;
- }
- } else
- goto out_err;
- out:
- return err;
- out_err:
- kmem_cache_destroy(sysfs_dir_cachep);
- sysfs_dir_cachep = NULL;
- goto out;
- }