Re: [PATCH] fs/dcache.c: re-add cond_resched() in shrink_dcache_parent()

Al Viro
2018-04-15T21:57:10Z

On Sun, Apr 15, 2018 at 09:40:54PM +0100, Al Viro wrote:

> BTW, the current placement of cond_resched() looks bogus; suppose we
> have collected a lot of victims and ran into need_resched(). We leave
> d_walk() and call shrink_dentry_list(). At that point there's a lot
> of stuff on our shrink list and anybody else running into them will
> have to keep scanning. Giving up the timeslice before we take care
> of any of those looks like a bad idea, to put it mildly, and that's
> precisely what will happen.
>
> What about doing that in the end of __dentry_kill() instead? And to
> hell with both existing call sites - dput() one (before going to
> the parent) is obviously covered by that (dentry_kill() only returns
> non-NULL after having called __dentry_kill()) and in shrink_dentry_list()
> we'll get to it as soon as we go through all dentries that can be
> immediately kicked off the shrink list. Which, AFAICS, improves the
> situation, now that shrink_lock_dentry() contains no trylock loops...
>
> Comments?

What I mean is something like this (cumulative diff, it'll obviously need
to be carved up into 3--4 commits):

diff --git a/fs/dcache.c b/fs/dcache.c
index 86d2de63461e..b1e62b54941b 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -580,6 +580,7 @@ static void __dentry_kill(struct dentry *dentry)
spin_unlock(&dentry->d_lock);
if (likely(can_free))
dentry_free(dentry);
+ cond_resched();
}

static struct dentry *__lock_parent(struct dentry *dentry)
@@ -827,30 +828,23 @@ static inline bool fast_dput(struct dentry *dentry)
*/
void dput(struct dentry *dentry)
{
- if (unlikely(!dentry))
- return;
+ while (dentry) {
+ might_sleep();

-repeat:
- might_sleep();
+ rcu_read_lock();
+ if (likely(fast_dput(dentry))) {
+ rcu_read_unlock();
+ return;
+ }

- rcu_read_lock();
- if (likely(fast_dput(dentry))) {
+ /* Slow case: now with the dentry lock held */
rcu_read_unlock();
- return;
- }
-
- /* Slow case: now with the dentry lock held */
- rcu_read_unlock();
-
- if (likely(retain_dentry(dentry))) {
- spin_unlock(&dentry->d_lock);
- return;
- }

- dentry = dentry_kill(dentry);
- if (dentry) {
- cond_resched();
- goto repeat;
+ if (likely(retain_dentry(dentry))) {
+ spin_unlock(&dentry->d_lock);
+ return;
+ }
+ dentry = dentry_kill(dentry);
}
}
EXPORT_SYMBOL(dput);
@@ -1052,8 +1046,6 @@ static void shrink_dentry_list(struct list_head *list)
while (!list_empty(list)) {
struct dentry *dentry, *parent;

- cond_resched();
-
dentry = list_entry(list->prev, struct dentry, d_lru);
spin_lock(&dentry->d_lock);
rcu_read_lock();
@@ -1230,13 +1222,11 @@ enum d_walk_ret {
* @parent: start of walk
* @data: data passed to @enter() and @finish()
* @enter: callback when first entering the dentry
- * @finish: callback when successfully finished the walk
*
* The @enter() and @finish() callbacks are called with d_lock held.
*/
static void d_walk(struct dentry *parent, void *data,
- enum d_walk_ret (*enter)(void *, struct dentry *),
- void (*finish)(void *))
+ enum d_walk_ret (*enter)(void *, struct dentry *))
{
struct dentry *this_parent;
struct list_head *next;
@@ -1325,8 +1315,6 @@ static void d_walk(struct dentry *parent, void *data,
if (need_seqretry(&rename_lock, seq))
goto rename_retry;
rcu_read_unlock();
- if (finish)
- finish(data);

out_unlock:
spin_unlock(&this_parent->d_lock);
@@ -1375,7 +1363,7 @@ int path_has_submounts(const struct path *parent)
struct check_mount data = { .mnt = parent->mnt, .mounted = 0 };

read_seqlock_excl(&mount_lock);
- d_walk(parent->dentry, &data, path_check_mount, NULL);
+ d_walk(parent->dentry, &data, path_check_mount);
read_sequnlock_excl(&mount_lock);

return data.mounted;
@@ -1483,7 +1471,7 @@ void shrink_dcache_parent(struct dentry *parent)
data.start = parent;
data.found = 0;

- d_walk(parent, &data, select_collect, NULL);
+ d_walk(parent, &data, select_collect);
if (!data.found)
break;

@@ -1518,7 +1506,7 @@ static enum d_walk_ret umount_check(void *_data, struct dentry *dentry)
static void do_one_tree(struct dentry *dentry)
{
shrink_dcache_parent(dentry);
- d_walk(dentry, dentry, umount_check, NULL);
+ d_walk(dentry, dentry, umount_check);
d_drop(dentry);
dput(dentry);
}
@@ -1542,78 +1530,49 @@ void shrink_dcache_for_umount(struct super_block *sb)
}
}

-struct detach_data {
- struct select_data select;
- struct dentry *mountpoint;
-};
-static enum d_walk_ret detach_and_collect(void *_data, struct dentry *dentry)
+static enum d_walk_ret find_submount(void *_data, struct dentry *dentry)
{
- struct detach_data *data = _data;
-
+ struct dentry **victim = (struct dentry **)_data;
if (d_mountpoint(dentry)) {
__dget_dlock(dentry);
- data->mountpoint = dentry;
+ *victim = dentry;
return D_WALK_QUIT;
}

- return select_collect(&data->select, dentry);
-}
-
-static void check_and_drop(void *_data)
-{
- struct detach_data *data = _data;
-
- if (!data->mountpoint && list_empty(&data->select.dispose))
- __d_drop(data->select.start);
+ return D_WALK_CONTINUE;
}

/**
* d_invalidate - detach submounts, prune dcache, and drop
* @dentry: dentry to invalidate (aka detach, prune and drop)
- *
- * no dcache lock.
- *
- * The final d_drop is done as an atomic operation relative to
- * rename_lock ensuring there are no races with d_set_mounted. This
- * ensures there are no unhashed dentries on the path to a mountpoint.
*/
void d_invalidate(struct dentry *dentry)
{
- /*
- * If it's already been dropped, return OK.
- */
+ bool had_submounts = false;
spin_lock(&dentry->d_lock);
if (d_unhashed(dentry)) {
spin_unlock(&dentry->d_lock);
return;
}
+ __d_drop(dentry);
spin_unlock(&dentry->d_lock);

/* Negative dentries can be dropped without further checks */
- if (!dentry->d_inode) {
- d_drop(dentry);
+ if (!dentry->d_inode)
return;
- }

+ shrink_dcache_parent(dentry);
for (;;) {
- struct detach_data data;
-
- data.mountpoint = NULL;
- INIT_LIST_HEAD(&data.select.dispose);
- data.select.start = dentry;
- data.select.found = 0;
-
- d_walk(dentry, &data, detach_and_collect, check_and_drop);
-
- if (!list_empty(&data.select.dispose))
- shrink_dentry_list(&data.select.dispose);
- else if (!data.mountpoint)
+ struct dentry *victim = NULL;
+ d_walk(dentry, &victim, find_submount);
+ if (!victim) {
+ if (had_submounts)
+ shrink_dcache_parent(dentry);
return;
-
- if (data.mountpoint) {
- detach_mounts(data.mountpoint);
- dput(data.mountpoint);
}
+ had_submounts = true;
+ detach_mounts(victim);
+ dput(victim);
}
}
EXPORT_SYMBOL(d_invalidate);
@@ -3112,7 +3071,7 @@ static enum d_walk_ret d_genocide_kill(void *data, struct dentry *dentry)

void d_genocide(struct dentry *parent)
{
- d_walk(parent, parent, d_genocide_kill, NULL);
+ d_walk(parent, parent, d_genocide_kill);
}

EXPORT_SYMBOL(d_genocide);