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[PATCH] shrink dentry struct
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Some long time ago, dentry struct was carefully tuned so that on 32 bits
UP, sizeof(struct dentry) was exactly 128, ie a power of 2, and a multiple
of memory cache lines.

Then RCU was added and dentry struct enlarged by two pointers, with nice
results for SMP, but not so good on UP, because breaking the above tuning
(128 + 8 = 136 bytes)

This patch reverts this unwanted side effect, by using an union (d_u),
where d_rcu and d_child are placed so that these two fields can share their
memory needs.

At the time d_free() is called (and d_rcu is really used), d_child is known
to be empty and not touched by the dentry freeing.

Lockless lookups only access d_name, d_parent, d_lock, d_op, d_flags (so
the previous content of d_child is not needed if said dentry was unhashed
but still accessed by a CPU because of RCU constraints)

As dentry cache easily contains millions of entries, a size reduction is
worth the extra complexity of the ugly C union.

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Dipankar Sarma <[email protected]>
Cc: Maneesh Soni <[email protected]>
Cc: Miklos Szeredi <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Ian Kent <[email protected]>
Cc: Paul Jackson <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Trond Myklebust <[email protected]>
Cc: Neil Brown <[email protected]>
Cc: James Morris <[email protected]>
Cc: Stephen Smalley <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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Eric Dumazet authored and Linus Torvalds committed Jan 9, 2006
1 parent 21b6bf1 commit 5160ee6
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Showing 15 changed files with 54 additions and 48 deletions.
6 changes: 3 additions & 3 deletions drivers/usb/core/inode.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -186,7 +186,7 @@ static void update_bus(struct dentry *bus)

down(&bus->d_inode->i_sem);

list_for_each_entry(dev, &bus->d_subdirs, d_child)
list_for_each_entry(dev, &bus->d_subdirs, d_u.d_child)
if (dev->d_inode)
update_dev(dev);

Expand All @@ -203,7 +203,7 @@ static void update_sb(struct super_block *sb)

down(&root->d_inode->i_sem);

list_for_each_entry(bus, &root->d_subdirs, d_child) {
list_for_each_entry(bus, &root->d_subdirs, d_u.d_child) {
if (bus->d_inode) {
switch (S_IFMT & bus->d_inode->i_mode) {
case S_IFDIR:
Expand Down Expand Up @@ -319,7 +319,7 @@ static int usbfs_empty (struct dentry *dentry)
spin_lock(&dcache_lock);

list_for_each(list, &dentry->d_subdirs) {
struct dentry *de = list_entry(list, struct dentry, d_child);
struct dentry *de = list_entry(list, struct dentry, d_u.d_child);
if (usbfs_positive(de)) {
spin_unlock(&dcache_lock);
return 0;
Expand Down
2 changes: 1 addition & 1 deletion fs/autofs4/autofs_i.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -209,7 +209,7 @@ static inline int simple_empty_nolock(struct dentry *dentry)
struct dentry *child;
int ret = 0;

list_for_each_entry(child, &dentry->d_subdirs, d_child)
list_for_each_entry(child, &dentry->d_subdirs, d_u.d_child)
if (simple_positive(child))
goto out;
ret = 1;
Expand Down
12 changes: 6 additions & 6 deletions fs/autofs4/expire.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -105,7 +105,7 @@ static int autofs4_check_tree(struct vfsmount *mnt,
next = this_parent->d_subdirs.next;
resume:
while (next != &this_parent->d_subdirs) {
struct dentry *dentry = list_entry(next, struct dentry, d_child);
struct dentry *dentry = list_entry(next, struct dentry, d_u.d_child);

/* Negative dentry - give up */
if (!simple_positive(dentry)) {
Expand Down Expand Up @@ -138,7 +138,7 @@ static int autofs4_check_tree(struct vfsmount *mnt,
}

if (this_parent != top) {
next = this_parent->d_child.next;
next = this_parent->d_u.d_child.next;
this_parent = this_parent->d_parent;
goto resume;
}
Expand All @@ -163,7 +163,7 @@ static struct dentry *autofs4_check_leaves(struct vfsmount *mnt,
next = this_parent->d_subdirs.next;
resume:
while (next != &this_parent->d_subdirs) {
struct dentry *dentry = list_entry(next, struct dentry, d_child);
struct dentry *dentry = list_entry(next, struct dentry, d_u.d_child);

/* Negative dentry - give up */
if (!simple_positive(dentry)) {
Expand Down Expand Up @@ -199,7 +199,7 @@ static struct dentry *autofs4_check_leaves(struct vfsmount *mnt,
}

if (this_parent != parent) {
next = this_parent->d_child.next;
next = this_parent->d_u.d_child.next;
this_parent = this_parent->d_parent;
goto resume;
}
Expand Down Expand Up @@ -238,7 +238,7 @@ static struct dentry *autofs4_expire(struct super_block *sb,
/* On exit from the loop expire is set to a dgot dentry
* to expire or it's NULL */
while ( next != &root->d_subdirs ) {
struct dentry *dentry = list_entry(next, struct dentry, d_child);
struct dentry *dentry = list_entry(next, struct dentry, d_u.d_child);

/* Negative dentry - give up */
if ( !simple_positive(dentry) ) {
Expand Down Expand Up @@ -302,7 +302,7 @@ static struct dentry *autofs4_expire(struct super_block *sb,
expired, (int)expired->d_name.len, expired->d_name.name);
spin_lock(&dcache_lock);
list_del(&expired->d_parent->d_subdirs);
list_add(&expired->d_parent->d_subdirs, &expired->d_child);
list_add(&expired->d_parent->d_subdirs, &expired->d_u.d_child);
spin_unlock(&dcache_lock);
return expired;
}
Expand Down
4 changes: 2 additions & 2 deletions fs/autofs4/inode.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -91,7 +91,7 @@ static void autofs4_force_release(struct autofs_sb_info *sbi)
next = this_parent->d_subdirs.next;
resume:
while (next != &this_parent->d_subdirs) {
struct dentry *dentry = list_entry(next, struct dentry, d_child);
struct dentry *dentry = list_entry(next, struct dentry, d_u.d_child);

/* Negative dentry - don`t care */
if (!simple_positive(dentry)) {
Expand All @@ -117,7 +117,7 @@ static void autofs4_force_release(struct autofs_sb_info *sbi)
if (this_parent != sbi->root) {
struct dentry *dentry = this_parent;

next = this_parent->d_child.next;
next = this_parent->d_u.d_child.next;
this_parent = this_parent->d_parent;
spin_unlock(&dcache_lock);
DPRINTK("parent dentry %p %.*s",
Expand Down
3 changes: 2 additions & 1 deletion fs/autofs4/root.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -143,7 +143,8 @@ static int autofs4_dcache_readdir(struct file * filp, void * dirent, filldir_t f
}

while(1) {
struct dentry *de = list_entry(list, struct dentry, d_child);
struct dentry *de = list_entry(list,
struct dentry, d_u.d_child);

if (!d_unhashed(de) && de->d_inode) {
spin_unlock(&dcache_lock);
Expand Down
2 changes: 1 addition & 1 deletion fs/coda/cache.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -93,7 +93,7 @@ static void coda_flag_children(struct dentry *parent, int flag)
spin_lock(&dcache_lock);
list_for_each(child, &parent->d_subdirs)
{
de = list_entry(child, struct dentry, d_child);
de = list_entry(child, struct dentry, d_u.d_child);
/* don't know what to do with negative dentries */
if ( ! de->d_inode )
continue;
Expand Down
34 changes: 17 additions & 17 deletions fs/dcache.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -71,7 +71,7 @@ struct dentry_stat_t dentry_stat = {

static void d_callback(struct rcu_head *head)
{
struct dentry * dentry = container_of(head, struct dentry, d_rcu);
struct dentry * dentry = container_of(head, struct dentry, d_u.d_rcu);

if (dname_external(dentry))
kfree(dentry->d_name.name);
Expand All @@ -86,7 +86,7 @@ static void d_free(struct dentry *dentry)
{
if (dentry->d_op && dentry->d_op->d_release)
dentry->d_op->d_release(dentry);
call_rcu(&dentry->d_rcu, d_callback);
call_rcu(&dentry->d_u.d_rcu, d_callback);
}

/*
Expand Down Expand Up @@ -193,7 +193,7 @@ kill_it: {
list_del(&dentry->d_lru);
dentry_stat.nr_unused--;
}
list_del(&dentry->d_child);
list_del(&dentry->d_u.d_child);
dentry_stat.nr_dentry--; /* For d_free, below */
/*drops the locks, at that point nobody can reach this dentry */
dentry_iput(dentry);
Expand Down Expand Up @@ -367,7 +367,7 @@ static inline void prune_one_dentry(struct dentry * dentry)
struct dentry * parent;

__d_drop(dentry);
list_del(&dentry->d_child);
list_del(&dentry->d_u.d_child);
dentry_stat.nr_dentry--; /* For d_free, below */
dentry_iput(dentry);
parent = dentry->d_parent;
Expand Down Expand Up @@ -518,7 +518,7 @@ int have_submounts(struct dentry *parent)
resume:
while (next != &this_parent->d_subdirs) {
struct list_head *tmp = next;
struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
next = tmp->next;
/* Have we found a mount point ? */
if (d_mountpoint(dentry))
Expand All @@ -532,7 +532,7 @@ int have_submounts(struct dentry *parent)
* All done at this level ... ascend and resume the search.
*/
if (this_parent != parent) {
next = this_parent->d_child.next;
next = this_parent->d_u.d_child.next;
this_parent = this_parent->d_parent;
goto resume;
}
Expand Down Expand Up @@ -569,7 +569,7 @@ static int select_parent(struct dentry * parent)
resume:
while (next != &this_parent->d_subdirs) {
struct list_head *tmp = next;
struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
next = tmp->next;

if (!list_empty(&dentry->d_lru)) {
Expand Down Expand Up @@ -610,7 +610,7 @@ dentry->d_parent->d_name.name, dentry->d_name.name, found);
* All done at this level ... ascend and resume the search.
*/
if (this_parent != parent) {
next = this_parent->d_child.next;
next = this_parent->d_u.d_child.next;
this_parent = this_parent->d_parent;
#ifdef DCACHE_DEBUG
printk(KERN_DEBUG "select_parent: ascending to %s/%s, found=%d\n",
Expand Down Expand Up @@ -753,12 +753,12 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
dentry->d_parent = dget(parent);
dentry->d_sb = parent->d_sb;
} else {
INIT_LIST_HEAD(&dentry->d_child);
INIT_LIST_HEAD(&dentry->d_u.d_child);
}

spin_lock(&dcache_lock);
if (parent)
list_add(&dentry->d_child, &parent->d_subdirs);
list_add(&dentry->d_u.d_child, &parent->d_subdirs);
dentry_stat.nr_dentry++;
spin_unlock(&dcache_lock);

Expand Down Expand Up @@ -1310,8 +1310,8 @@ void d_move(struct dentry * dentry, struct dentry * target)
/* Unhash the target: dput() will then get rid of it */
__d_drop(target);

list_del(&dentry->d_child);
list_del(&target->d_child);
list_del(&dentry->d_u.d_child);
list_del(&target->d_u.d_child);

/* Switch the names.. */
switch_names(dentry, target);
Expand All @@ -1322,15 +1322,15 @@ void d_move(struct dentry * dentry, struct dentry * target)
if (IS_ROOT(dentry)) {
dentry->d_parent = target->d_parent;
target->d_parent = target;
INIT_LIST_HEAD(&target->d_child);
INIT_LIST_HEAD(&target->d_u.d_child);
} else {
do_switch(dentry->d_parent, target->d_parent);

/* And add them back to the (new) parent lists */
list_add(&target->d_child, &target->d_parent->d_subdirs);
list_add(&target->d_u.d_child, &target->d_parent->d_subdirs);
}

list_add(&dentry->d_child, &dentry->d_parent->d_subdirs);
list_add(&dentry->d_u.d_child, &dentry->d_parent->d_subdirs);
spin_unlock(&target->d_lock);
spin_unlock(&dentry->d_lock);
write_sequnlock(&rename_lock);
Expand Down Expand Up @@ -1568,7 +1568,7 @@ void d_genocide(struct dentry *root)
resume:
while (next != &this_parent->d_subdirs) {
struct list_head *tmp = next;
struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
next = tmp->next;
if (d_unhashed(dentry)||!dentry->d_inode)
continue;
Expand All @@ -1579,7 +1579,7 @@ void d_genocide(struct dentry *root)
atomic_dec(&dentry->d_count);
}
if (this_parent != root) {
next = this_parent->d_child.next;
next = this_parent->d_u.d_child.next;
atomic_dec(&this_parent->d_count);
this_parent = this_parent->d_parent;
goto resume;
Expand Down
12 changes: 6 additions & 6 deletions fs/libfs.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -93,16 +93,16 @@ loff_t dcache_dir_lseek(struct file *file, loff_t offset, int origin)
loff_t n = file->f_pos - 2;

spin_lock(&dcache_lock);
list_del(&cursor->d_child);
list_del(&cursor->d_u.d_child);
p = file->f_dentry->d_subdirs.next;
while (n && p != &file->f_dentry->d_subdirs) {
struct dentry *next;
next = list_entry(p, struct dentry, d_child);
next = list_entry(p, struct dentry, d_u.d_child);
if (!d_unhashed(next) && next->d_inode)
n--;
p = p->next;
}
list_add_tail(&cursor->d_child, p);
list_add_tail(&cursor->d_u.d_child, p);
spin_unlock(&dcache_lock);
}
}
Expand All @@ -126,7 +126,7 @@ int dcache_readdir(struct file * filp, void * dirent, filldir_t filldir)
{
struct dentry *dentry = filp->f_dentry;
struct dentry *cursor = filp->private_data;
struct list_head *p, *q = &cursor->d_child;
struct list_head *p, *q = &cursor->d_u.d_child;
ino_t ino;
int i = filp->f_pos;

Expand All @@ -153,7 +153,7 @@ int dcache_readdir(struct file * filp, void * dirent, filldir_t filldir)
}
for (p=q->next; p != &dentry->d_subdirs; p=p->next) {
struct dentry *next;
next = list_entry(p, struct dentry, d_child);
next = list_entry(p, struct dentry, d_u.d_child);
if (d_unhashed(next) || !next->d_inode)
continue;

Expand Down Expand Up @@ -261,7 +261,7 @@ int simple_empty(struct dentry *dentry)
int ret = 0;

spin_lock(&dcache_lock);
list_for_each_entry(child, &dentry->d_subdirs, d_child)
list_for_each_entry(child, &dentry->d_subdirs, d_u.d_child)
if (simple_positive(child))
goto out;
ret = 1;
Expand Down
2 changes: 1 addition & 1 deletion fs/ncpfs/dir.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -365,7 +365,7 @@ ncp_dget_fpos(struct dentry *dentry, struct dentry *parent, unsigned long fpos)
spin_lock(&dcache_lock);
next = parent->d_subdirs.next;
while (next != &parent->d_subdirs) {
dent = list_entry(next, struct dentry, d_child);
dent = list_entry(next, struct dentry, d_u.d_child);
if ((unsigned long)dent->d_fsdata == fpos) {
if (dent->d_inode)
dget_locked(dent);
Expand Down
4 changes: 2 additions & 2 deletions fs/ncpfs/ncplib_kernel.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -196,7 +196,7 @@ ncp_renew_dentries(struct dentry *parent)
spin_lock(&dcache_lock);
next = parent->d_subdirs.next;
while (next != &parent->d_subdirs) {
dentry = list_entry(next, struct dentry, d_child);
dentry = list_entry(next, struct dentry, d_u.d_child);

if (dentry->d_fsdata == NULL)
ncp_age_dentry(server, dentry);
Expand All @@ -218,7 +218,7 @@ ncp_invalidate_dircache_entries(struct dentry *parent)
spin_lock(&dcache_lock);
next = parent->d_subdirs.next;
while (next != &parent->d_subdirs) {
dentry = list_entry(next, struct dentry, d_child);
dentry = list_entry(next, struct dentry, d_u.d_child);
dentry->d_fsdata = NULL;
ncp_age_dentry(server, dentry);
next = next->next;
Expand Down
4 changes: 2 additions & 2 deletions fs/smbfs/cache.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -66,7 +66,7 @@ smb_invalidate_dircache_entries(struct dentry *parent)
spin_lock(&dcache_lock);
next = parent->d_subdirs.next;
while (next != &parent->d_subdirs) {
dentry = list_entry(next, struct dentry, d_child);
dentry = list_entry(next, struct dentry, d_u.d_child);
dentry->d_fsdata = NULL;
smb_age_dentry(server, dentry);
next = next->next;
Expand Down Expand Up @@ -100,7 +100,7 @@ smb_dget_fpos(struct dentry *dentry, struct dentry *parent, unsigned long fpos)
spin_lock(&dcache_lock);
next = parent->d_subdirs.next;
while (next != &parent->d_subdirs) {
dent = list_entry(next, struct dentry, d_child);
dent = list_entry(next, struct dentry, d_u.d_child);
if ((unsigned long)dent->d_fsdata == fpos) {
if (dent->d_inode)
dget_locked(dent);
Expand Down
9 changes: 7 additions & 2 deletions include/linux/dcache.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -95,14 +95,19 @@ struct dentry {
struct qstr d_name;

struct list_head d_lru; /* LRU list */
struct list_head d_child; /* child of parent list */
/*
* d_child and d_rcu can share memory
*/
union {
struct list_head d_child; /* child of parent list */
struct rcu_head d_rcu;
} d_u;
struct list_head d_subdirs; /* our children */
struct list_head d_alias; /* inode alias list */
unsigned long d_time; /* used by d_revalidate */
struct dentry_operations *d_op;
struct super_block *d_sb; /* The root of the dentry tree */
void *d_fsdata; /* fs-specific data */
struct rcu_head d_rcu;
struct dcookie_struct *d_cookie; /* cookie, if any */
int d_mounted;
unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /* small names */
Expand Down
4 changes: 2 additions & 2 deletions kernel/cpuset.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -331,7 +331,7 @@ static void cpuset_d_remove_dir(struct dentry *dentry)
spin_lock(&dcache_lock);
node = dentry->d_subdirs.next;
while (node != &dentry->d_subdirs) {
struct dentry *d = list_entry(node, struct dentry, d_child);
struct dentry *d = list_entry(node, struct dentry, d_u.d_child);
list_del_init(node);
if (d->d_inode) {
d = dget_locked(d);
Expand All @@ -343,7 +343,7 @@ static void cpuset_d_remove_dir(struct dentry *dentry)
}
node = dentry->d_subdirs.next;
}
list_del_init(&dentry->d_child);
list_del_init(&dentry->d_u.d_child);
spin_unlock(&dcache_lock);
remove_dir(dentry);
}
Expand Down
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