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/string.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/malloc.h>
#include <linux/slab.h>
#include <linux/init.h>

#include <asm/uaccess.h>

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

/* For managing the dcache */
extern unsigned long num_physpages, page_cache_size;
extern int inodes_stat[];
#define nr_inodes (inodes_stat[0])

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     10
#define D_HASHSIZE     (1UL << D_HASHBITS)
#define D_HASHMASK     (D_HASHSIZE-1)

static struct list_head dentry_hashtable[D_HASHSIZE];
static LIST_HEAD(dentry_unused);

struct {
	int nr_dentry;
	int nr_unused;
	int age_limit;		/* age in seconds */
	int want_pages;		/* pages requested by system */
	int dummy[2];
} dentry_stat = {0, 0, 45, 0,};

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); 
}

/*
 * Release the dentry's inode, using the fileystem
 * d_iput() operation if defined.
 */
static inline void dentry_iput(struct dentry * dentry)
{
	struct inode *inode = dentry->d_inode;
	if (inode) {
		dentry->d_inode = NULL;
		list_del(&dentry->d_alias);
		INIT_LIST_HEAD(&dentry->d_alias);
		if (dentry->d_op && dentry->d_op->d_iput)
			dentry->d_op->d_iput(dentry, inode);
		else
			iput(inode);
	}
}

/*
 * 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.
 */
void dput(struct dentry *dentry)
{
	int count;

	if (!dentry)
		return;

repeat:
	count = dentry->d_count - 1;
	if (count != 0)
		goto out;

	/*
	 * Note that if d_op->d_delete blocks,
	 * the dentry could go back in use.
	 * Each fs will have to watch for this.
	 */
	if (dentry->d_op && dentry->d_op->d_delete) {
		dentry->d_op->d_delete(dentry);

		count = dentry->d_count - 1;
		if (count != 0)
			goto out;
	}

	if (!list_empty(&dentry->d_lru)) {
		dentry_stat.nr_unused--;
		list_del(&dentry->d_lru);
	}
	if (list_empty(&dentry->d_hash)) {
		struct dentry * parent;

		list_del(&dentry->d_child);
		dentry_iput(dentry);
		parent = dentry->d_parent;
		d_free(dentry);
		if (dentry == parent)
			return;
		dentry = parent;
		goto repeat;
	}
	list_add(&dentry->d_lru, &dentry_unused);
	dentry_stat.nr_unused++;
	/*
	 * Update the timestamp
	 */
	dentry->d_reftime = jiffies;

out:
	if (count >= 0) {
		dentry->d_count = count;
		return;
	}

	printk(KERN_CRIT "Negative d_count (%d) for %s/%s\n",
		count,
		dentry->d_parent->d_name.name,
		dentry->d_name.name);
	*(int *)0 = 0;	
}

/*
 * 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.
 */
int d_invalidate(struct dentry * dentry)
{
	/*
	 * Check whether to do a partial shrink_dcache
	 * to get rid of unused child entries.
	 */
	if (!list_empty(&dentry->d_subdirs)) {
		shrink_dcache_parent(dentry);
	}

	/*
	 * 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 (dentry->d_count > 1) {
		if (dentry->d_inode && S_ISDIR(dentry->d_inode->i_mode))
			return -EBUSY;
	}

	d_drop(dentry);
	return 0;
}

/*
 * Select less valuable dentries to be pruned when we need
 * inodes or memory. The selected dentries are moved to the
 * old end of the list where prune_dcache() can find them.
 * 
 * Negative dentries are included in the selection so that
 * they don't accumulate at the end of the list. The count
 * returned is the total number of dentries selected, which
 * may be much larger than the requested number of inodes.
 */
int select_dcache(int inode_count, int page_count)
{
	struct list_head *next, *tail = &dentry_unused;
	int found = 0;
	int depth = dentry_stat.nr_unused >> 1;
	unsigned long max_value = 4;

	if (page_count)
		max_value = -1;

	next = tail->prev;
	while (next != &dentry_unused && depth--) {
		struct list_head *tmp = next;
		struct dentry *dentry = list_entry(tmp, struct dentry, d_lru);
		struct inode *inode = dentry->d_inode;
		unsigned long value = 0;	

		next = tmp->prev;
		if (dentry->d_count) {
			dentry_stat.nr_unused--;
			list_del(tmp);
			INIT_LIST_HEAD(tmp);
			continue;
		}

		/*
		 * Select dentries based on the page cache count ...
		 * should factor in number of uses as well. We take
		 * all negative dentries so that they don't accumulate.
		 * (We skip inodes that aren't immediately available.)
		 */
		if (inode) {
			value = inode->i_nrpages;	
			if (value >= max_value)
				continue;
			if (inode->i_state || inode->i_count > 1)
				continue;
		}

		/*
		 * Move the selected dentries behind the tail.
		 */
		if (tmp != tail->prev) {
			list_del(tmp);
			list_add(tmp, tail->prev);
		}
		tail = tmp;
		found++;
		if (inode && --inode_count <= 0)
			break;
		if (page_count && (page_count -= value) <= 0)
			break;
	}
	return found;
}

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

	list_del(&dentry->d_hash);
	list_del(&dentry->d_child);
	dentry_iput(dentry);
	parent = dentry->d_parent;
	d_free(dentry);
	dput(parent);
}

/*
 * 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).
 */
void prune_dcache(int count)
{
	for (;;) {
		struct dentry *dentry;
		struct list_head *tmp = dentry_unused.prev;

		if (tmp == &dentry_unused)
			break;
		dentry_stat.nr_unused--;
		list_del(tmp);
		INIT_LIST_HEAD(tmp);
		dentry = list_entry(tmp, struct dentry, d_lru);
		if (!dentry->d_count) {
			prune_one_dentry(dentry);
			if (!--count)
				break;
		}
	}
}

/*
 * 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.
 */
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.
	 */
	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 (dentry->d_count)
			continue;
		dentry_stat.nr_unused--;
		list_del(tmp);
		INIT_LIST_HEAD(tmp);
		prune_one_dentry(dentry);
		goto repeat;
	}
}

/*
 * Check whether a root dentry would be in use if all of its
 * child dentries were freed. This allows a non-destructive
 * test for unmounting a device.
 */
int is_root_busy(struct dentry *root)
{
	struct dentry *this_parent = root;
	struct list_head *next;
	int count = root->d_count;

repeat:
	next = this_parent->d_subdirs.next;
resume:
	while (next != &this_parent->d_subdirs) {
		struct list_head *tmp = next;
		struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
		next = tmp->next;
		/* Decrement count for unused children */
		count += (dentry->d_count - 1);
		if (!list_empty(&dentry->d_subdirs)) {
			this_parent = dentry;
			goto repeat;
		}
		/* root is busy if any leaf is busy */
		if (dentry->d_count)
			return 1;
	}
	/*
	 * All done at this level ... ascend and resume the search.
	 */
	if (this_parent != root) {
		next = this_parent->d_child.next; 
		this_parent = this_parent->d_parent;
		goto resume;
	}
	return (count > 1); /* remaining users? */
}

/*
 * Search the dentry child list for the specified parent,
 * and move any unused dentries to the end of the unused
 * list for prune_dcache(). We descend to the next level
 * whenever the d_subdirs list is non-empty and continue
 * searching.
 */
static int select_parent(struct dentry * parent)
{
	struct dentry *this_parent = parent;
	struct list_head *next;
	int found = 0;

repeat:
	next = this_parent->d_subdirs.next;
resume:
	while (next != &this_parent->d_subdirs) {
		struct list_head *tmp = next;
		struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
		next = tmp->next;
		if (!dentry->d_count) {
			list_del(&dentry->d_lru);
			list_add(&dentry->d_lru, dentry_unused.prev);
			found++;
		}
		/*
		 * Descend a level if the d_subdirs list is non-empty.
		 */
		if (!list_empty(&dentry->d_subdirs)) {
			this_parent = dentry;
#ifdef DCACHE_DEBUG
printk(KERN_DEBUG "select_parent: descending to %s/%s, found=%d\n",
dentry->d_parent->d_name.name, dentry->d_name.name, found);
#endif
			goto repeat;
		}
	}
	/*
	 * All done at this level ... ascend and resume the search.
	 */
	if (this_parent != parent) {
		next = this_parent->d_child.next; 
		this_parent = this_parent->d_parent;
#ifdef DCACHE_DEBUG
printk(KERN_DEBUG "select_parent: ascending to %s/%s, found=%d\n",
this_parent->d_parent->d_name.name, this_parent->d_name.name, found);
#endif
		goto resume;
	}
	return found;
}

/*
 * Prune the dcache to remove unused children of the parent dentry.
 */
void shrink_dcache_parent(struct dentry * parent)
{
	int found;

	while ((found = select_parent(parent)) != 0)
		prune_dcache(found);
}

/*
 * This is called from kswapd when we think we need some
 * more memory, but aren't really sure how much. So we
 * carefully try to free a _bit_ of our dcache, but not
 * too much.
 *