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SubjectRe: [PATCH] fs/dcache.c: re-add cond_resched() in shrink_dcache_parent()
On Sun, Apr 15, 2018 at 03:39:44AM +0100, Al Viro wrote:

> I really wonder if we should just do the following in
> d_invalidate():
> * grab ->d_lock on victim, check if it's unhashed,
> unlock and bugger off if it is. Otherwise, unhash and unlock.
> >From that point on any d_set_mounted() in the subtree will
> fail.
> * shrink_dcache_parent() to reduce the subtree size.
> * go through the (hopefully shrunk) subtree, picking
> mountpoints. detach_mounts() for each of them.
> * shrink_dcache_parent() if any points had been
> encountered, to kick the now-unpinned stuff.
>
> As a side benefit, we could probably be gentler on rename_lock
> in d_set_mounted() after that change...

Something like this, IOW:

diff --git a/fs/dcache.c b/fs/dcache.c
index 86d2de63461e..e61c07ff2d95 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1230,13 +1230,11 @@ 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 +1323,6 @@ 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 +1371,7 @@ 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 +1479,7 @@ 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 +1514,7 @@ 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 +1538,48 @@ 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;
-
if (d_mountpoint(dentry)) {
__dget_dlock(dentry);
- data->mountpoint = dentry;
+ *(struct dentry **)_data = 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 +3078,7 @@ 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);
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