dcache.c

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/*
 * fs/dcache.c
 *
 * Complete reimplementation
 * (C) 1997 Thomas Schoebel-Theuer,
 * with heavy changes by Linus Torvalds
 */

/*
 * Notes on the allocation strategy:
 *
 * The dcache is a master of the icache - whenever a dcache entry
 * exists, the inode will always exist. "iput()" is done either when
 * the dcache entry is deleted or garbage collected.
 */

#include <linux/config.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/smp_lock.h>
#include <linux/cache.h>
#include <linux/module.h>

#include <asm/uaccess.h>

#define DCACHE_PARANOIA 1
/* #define DCACHE_DEBUG 1 */

spinlock_t dcache_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;

/* Right now the dcache depends on the kernel lock */
#define check_lock()	if (!kernel_locked()) BUG()

static kmem_cache_t *dentry_cache; 

/*
 * This is the single most critical data structure when it comes
 * to the dcache: the hashtable for lookups. Somebody should try
 * to make this good - I've just made it work.
 *
 * This hash-function tries to avoid losing too many bits of hash
 * information, yet avoid using a prime hash-size or similar.
 */
#define D_HASHBITS     d_hash_shift
#define D_HASHMASK     d_hash_mask

static unsigned int d_hash_mask;
static unsigned int d_hash_shift;
static struct list_head *dentry_hashtable;
static LIST_HEAD(dentry_unused);

/* Statistics gathering. */
struct dentry_stat_t dentry_stat = {0, 0, 45, 0,};

/* no dcache_lock, please */
static inline void d_free(struct dentry *dentry)
{
	if (dentry->d_op && dentry->d_op->d_release)
		dentry->d_op->d_release(dentry);
	if (dname_external(dentry)) 
		kfree(dentry->d_name.name);
	kmem_cache_free(dentry_cache, dentry); 
	dentry_stat.nr_dentry--;
}

/*
 * Release the dentry's inode, using the fileystem
 * d_iput() operation if defined.
 * Called with dcache_lock held, drops it.
 */
static inline void dentry_iput(struct dentry * dentry)
{
	struct inode *inode = dentry->d_inode;
	if (inode) {
		dentry->d_inode = NULL;
		list_del_init(&dentry->d_alias);
		spin_unlock(&dcache_lock);
		if (dentry->d_op && dentry->d_op->d_iput)
			dentry->d_op->d_iput(dentry, inode);
		else
			iput(inode);
	} else
		spin_unlock(&dcache_lock);
}

/* 
 * This is dput
 *
 * This is complicated by the fact that we do not want to put
 * dentries that are no longer on any hash chain on the unused
 * list: we'd much rather just get rid of them immediately.
 *
 * However, that implies that we have to traverse the dentry
 * tree upwards to the parents which might _also_ now be
 * scheduled for deletion (it may have been only waiting for
 * its last child to go away).
 *
 * This tail recursion is done by hand as we don't want to depend
 * on the compiler to always get this right (gcc generally doesn't).
 * Real recursion would eat up our stack space.
 */

/*
 * dput - release a dentry
 * @dentry: dentry to release 
 *
 * Release a dentry. This will drop the usage count and if appropriate
 * call the dentry unlink method as well as removing it from the queues and
 * releasing its resources. If the parent dentries were scheduled for release
 * they too may now get deleted.
 *
 * no dcache lock, please.
 */

void dput(struct dentry *dentry)
{
	if (!dentry)
		return;

repeat:
	if (!atomic_dec_and_lock(&dentry->d_count, &dcache_lock))
		return;

	/* dput on a free dentry? */
	if (!list_empty(&dentry->d_lru))
		BUG();
	/*
	 * AV: ->d_delete() is _NOT_ allowed to block now.
	 */
	if (dentry->d_op && dentry->d_op->d_delete) {
		if (dentry->d_op->d_delete(dentry))
			goto unhash_it;
	}
	/* Unreachable? Get rid of it */
	if (list_empty(&dentry->d_hash))
		goto kill_it;
	list_add(&dentry->d_lru, &dentry_unused);
	dentry_stat.nr_unused++;
	spin_unlock(&dcache_lock);
	return;

unhash_it:
	list_del_init(&dentry->d_hash);

kill_it: {
		struct dentry *parent;
		list_del(&dentry->d_child);
		/* drops the lock, at that point nobody can reach this dentry */
		dentry_iput(dentry);
		parent = dentry->d_parent;
		d_free(dentry);
		if (dentry == parent)
			return;
		dentry = parent;
		goto repeat;
	}
}

/**
 * d_invalidate - invalidate a dentry
 * @dentry: dentry to invalidate
 *
 * Try to invalidate the dentry if it turns out to be
 * possible. If there are other dentries that can be
 * reached through this one we can't delete it and we
 * return -EBUSY. On success we return 0.
 *
 * no dcache lock.
 */
 
int d_invalidate(struct dentry * dentry)
{
	/*
	 * If it's already been dropped, return OK.
	 */
	spin_lock(&dcache_lock);
	if (list_empty(&dentry->d_hash)) {
		spin_unlock(&dcache_lock);
		return 0;
	}
	/*
	 * Check whether to do a partial shrink_dcache
	 * to get rid of unused child entries.
	 */
	if (!list_empty(&dentry->d_subdirs)) {
		spin_unlock(&dcache_lock);
		shrink_dcache_parent(dentry);
		spin_lock(&dcache_lock);
	}

	/*
	 * Somebody else still using it?
	 *
	 * If it's a directory, we can't drop it
	 * for fear of somebody re-populating it
	 * with children (even though dropping it
	 * would make it unreachable from the root,
	 * we might still populate it if it was a
	 * working directory or similar).
	 */
	if (atomic_read(&dentry->d_count) > 1) {
		if (dentry->d_inode && S_ISDIR(dentry->d_inode->i_mode)) {
			spin_unlock(&dcache_lock);
			return -EBUSY;
		}
	}

	list_del_init(&dentry->d_hash);
	spin_unlock(&dcache_lock);
	return 0;
}

/* This should be called _only_ with dcache_lock held */

static inline struct dentry * __dget_locked(struct dentry *dentry)
{
	atomic_inc(&dentry->d_count);
	if (atomic_read(&dentry->d_count) == 1) {
		dentry_stat.nr_unused--;
		list_del_init(&dentry->d_lru);
	}
	return dentry;
}

struct dentry * dget_locked(struct dentry *dentry)
{
	return __dget_locked(dentry);
}

/**
 * d_find_alias - grab a hashed alias of inode
 * @inode: inode in question
 *
 * If inode has a hashed alias - acquire the reference to alias and
 * return it. Otherwise return NULL. Notice that if inode is a directory
 * there can be only one alias and it can be unhashed only if it has
 * no children.
 */

struct dentry * d_find_alias(struct inode *inode)
{
	struct list_head *head, *next, *tmp;
	struct dentry *alias;

	spin_lock(&dcache_lock);
	head = &inode->i_dentry;
	next = inode->i_dentry.next;
	while (next != head) {
		tmp = next;
		next = tmp->next;
		alias = list_entry(tmp, struct dentry, d_alias);
		if (!list_empty(&alias->d_hash)) {
			__dget_locked(alias);
			spin_unlock(&dcache_lock);
			return alias;
		}
	}
	spin_unlock(&dcache_lock);
	return NULL;
}

/*
 *	Try to kill dentries associated with this inode.
 * WARNING: you must own a reference to inode.
 */
void d_prune_aliases(struct inode *inode)
{
	struct list_head *tmp, *head = &inode->i_dentry;
restart:
	spin_lock(&dcache_lock);
	tmp = head;
	while ((tmp = tmp->next) != head) {
		struct dentry *dentry = list_entry(tmp, struct dentry, d_alias);
		if (!atomic_read(&dentry->d_count)) {
			__dget_locked(dentry);
			spin_unlock(&dcache_lock);
			d_drop(dentry);
			dput(dentry);
			goto restart;
		}
	}
	spin_unlock(&dcache_lock);
}

/*
 * Throw away a dentry - free the inode, dput the parent.
 * This requires that the LRU list has already been
 * removed.
 * Called with dcache_lock, drops it and then regains.
 */
static inline void prune_one_dentry(struct dentry * dentry)
{
	struct dentry * parent;

	list_del_init(&dentry->d_hash);
	list_del(&dentry->d_child);
	dentry_iput(dentry);
	parent = dentry->d_parent;
	d_free(dentry);
	if (parent != dentry)
		dput(parent);
	spin_lock(&dcache_lock);
}

/**
 * prune_dcache - shrink the dcache
 * @count: number of entries to try and 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.
 */
 
void prune_dcache(int count)
{
	spin_lock(&dcache_lock);
	for (;;) {
		struct dentry *dentry;
		struct list_head *tmp;

		tmp = dentry_unused.prev;

		if (tmp == &dentry_unused)
			break;
		list_del_init(tmp);
		dentry = list_entry(tmp, struct dentry, d_lru);

		/* If the dentry was recently referenced, don't free it. */
		if (dentry->d_vfs_flags & DCACHE_REFERENCED) {
			dentry->d_vfs_flags &= ~DCACHE_REFERENCED;
			list_add(&dentry->d_lru, &dentry_unused);
			continue;
		}
		dentry_stat.nr_unused--;

		/* Unused dentry with a count? */
		if (atomic_read(&dentry->d_count))
			BUG();

		prune_one_dentry(dentry);
		if (!--count)
			break;
	}
	spin_unlock(&dcache_lock);
}

/*
 * Shrink the dcache for the specified super block.
 * This allows us to unmount a device without disturbing
 * the dcache for the other devices.
 *
 * This implementation makes just two traversals of the
 * unused list.  On the first pass we move the selected
 * dentries to the most recent end, and on the second
 * pass we free them.  The second pass must restart after
 * each dput(), but since the target dentries are all at
 * the end, it's really just a single traversal.
 */

/**
 * shrink_dcache_sb - shrink dcache for a superblock
 * @sb: superblock
 *
 * Shrink the dcache for the specified super block. This
 * is used to free the dcache before unmounting a file
 * system
 */

void shrink_dcache_sb(struct super_block * sb)
{
	struct list_head *tmp, *next;
	struct dentry *dentry;

	/*
	 * Pass one ... move the dentries for the specified
	 * superblock to the most recent end of the unused list.
	 */
	spin_lock(&dcache_lock);
	next = dentry_unused.next;
	while (next != &dentry_unused) {
		tmp = next;
		next = tmp->next;
		dentry = list_entry(tmp, struct dentry, d_lru);
		if (dentry->d_sb != sb)
			continue;
		list_del(tmp);
		list_add(tmp, &dentry_unused);
	}

	/*
	 * Pass two ... free the dentries for this superblock.
	 */
repeat:
	next = dentry_unused.next;
	while (next != &dentry_unused) {
		tmp = next;
		next = tmp->next;
		dentry = list_entry(tmp, struct dentry, d_lru);
		if (dentry->d_sb != sb)
			continue;
		if (atomic_read(&dentry->d_count))
			continue;
		dentry_stat.nr_unused--;
		list_del_init(tmp);
		prune_one_dentry(dentry);
		goto repeat;
	}
	spin_unlock(&dcache_lock);
}

/*
 * Search for at least 1 mount point in the dentry's subdirs.
 * We descend to the next level whenever the d_subdirs
 * list is non-empty and continue searching.
 */
 
/**
 * have_submounts - check for mounts over a dentry
 * @parent: dentry to check.
 *
 * Return true if the parent or its subdirectories contain
 * a mount point
 */