[PATCH v4 1/5] fsnotify: clear PARENT_WATCHED flags lazily

Stephen Brennan
2022-11-11T22:06:46Z

From: Amir Goldstein <amir73il@gmail.com>

Call fsnotify_update_children_dentry_flags() to set PARENT_WATCHED flags
only when parent starts watching children.

When parent stops watching children, clear false positive PARENT_WATCHED
flags lazily in __fsnotify_parent() for each accessed child.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
---

Notes:
This is Amir's patch based on Jan's idea, with no changes or alterations
from my side.

fs/notify/fsnotify.c | 26 ++++++++++++++++++++------
fs/notify/fsnotify.h | 3 ++-
fs/notify/mark.c | 32 +++++++++++++++++++++++++++++---
include/linux/fsnotify_backend.h | 8 +++++---
4 files changed, 56 insertions(+), 13 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 7974e91ffe13..2c50e9e50d35 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -103,17 +103,13 @@ void fsnotify_sb_delete(struct super_block *sb)
* parent cares. Thus when an event happens on a child it can quickly tell
* if there is a need to find a parent and send the event to the parent.
*/
-void __fsnotify_update_child_dentry_flags(struct inode *inode)
+void fsnotify_update_children_dentry_flags(struct inode *inode, bool watched)
{
struct dentry *alias;
- int watched;

if (!S_ISDIR(inode->i_mode))
return;

- /* determine if the children should tell inode about their events */
- watched = fsnotify_inode_watches_children(inode);
-
spin_lock(&inode->i_lock);
/* run all of the dentries associated with this inode. Since this is a
* directory, there damn well better only be one item on this list */
@@ -140,6 +136,24 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
spin_unlock(&inode->i_lock);
}

+/*
+ * Lazily clear false positive PARENT_WATCHED flag for child whose parent had
+ * stopped wacthing children.
+ */
+static void fsnotify_update_child_dentry_flags(struct inode *inode,
+ struct dentry *dentry)
+{
+ spin_lock(&dentry->d_lock);
+ /*
+ * d_lock is a sufficient barrier to prevent observing a non-watched
+ * parent state from before the fsnotify_update_children_dentry_flags()
+ * or fsnotify_update_flags() call that had set PARENT_WATCHED.
+ */
+ if (!fsnotify_inode_watches_children(inode))
+ dentry->d_flags &= ~DCACHE_FSNOTIFY_PARENT_WATCHED;
+ spin_unlock(&dentry->d_lock);
+}
+
/* Are inode/sb/mount interested in parent and name info with this event? */
static bool fsnotify_event_needs_parent(struct inode *inode, struct mount *mnt,
__u32 mask)
@@ -208,7 +222,7 @@ int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
p_inode = parent->d_inode;
p_mask = fsnotify_inode_watches_children(p_inode);
if (unlikely(parent_watched && !p_mask))
- __fsnotify_update_child_dentry_flags(p_inode);
+ fsnotify_update_child_dentry_flags(p_inode, dentry);

/*
* Include parent/name in notification either if some notification
diff --git a/fs/notify/fsnotify.h b/fs/notify/fsnotify.h
index fde74eb333cc..bce9be36d06b 100644
--- a/fs/notify/fsnotify.h
+++ b/fs/notify/fsnotify.h
@@ -74,7 +74,8 @@ static inline void fsnotify_clear_marks_by_sb(struct super_block *sb)
* update the dentry->d_flags of all of inode's children to indicate if inode cares
* about events that happen to its children.
*/
-extern void __fsnotify_update_child_dentry_flags(struct inode *inode);
+extern void fsnotify_update_children_dentry_flags(struct inode *inode,
+ bool watched);

extern struct kmem_cache *fsnotify_mark_connector_cachep;

diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index c74ef947447d..6797a2952f87 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -176,6 +176,24 @@ static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
return fsnotify_update_iref(conn, want_iref);
}

+static bool fsnotify_conn_watches_children(
+ struct fsnotify_mark_connector *conn)
+{
+ if (conn->type != FSNOTIFY_OBJ_TYPE_INODE)
+ return false;
+
+ return fsnotify_inode_watches_children(fsnotify_conn_inode(conn));
+}
+
+static void fsnotify_conn_set_children_dentry_flags(
+ struct fsnotify_mark_connector *conn)
+{
+ if (conn->type != FSNOTIFY_OBJ_TYPE_INODE)
+ return;
+
+ fsnotify_update_children_dentry_flags(fsnotify_conn_inode(conn), true);
+}
+
/*
* Calculate mask of events for a list of marks. The caller must make sure
* connector and connector->obj cannot disappear under us. Callers achieve
@@ -184,15 +202,23 @@ static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
*/
void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
{
+ bool update_children;
+
if (!conn)
return;

spin_lock(&conn->lock);
+ update_children = !fsnotify_conn_watches_children(conn);
__fsnotify_recalc_mask(conn);
+ update_children &= fsnotify_conn_watches_children(conn);
spin_unlock(&conn->lock);
- if (conn->type == FSNOTIFY_OBJ_TYPE_INODE)
- __fsnotify_update_child_dentry_flags(
- fsnotify_conn_inode(conn));
+ /*
+ * Set children's PARENT_WATCHED flags only if parent started watching.
+ * When parent stops watching, we clear false positive PARENT_WATCHED
+ * flags lazily in __fsnotify_parent().
+ */
+ if (update_children)
+ fsnotify_conn_set_children_dentry_flags(conn);
}

/* Free all connectors queued for freeing once SRCU period ends */
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index d7d96c806bff..1276de409724 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -563,12 +563,14 @@ static inline __u32 fsnotify_parent_needed_mask(__u32 mask)

static inline int fsnotify_inode_watches_children(struct inode *inode)
{
+ __u32 parent_mask = READ_ONCE(inode->i_fsnotify_mask);
+
/* FS_EVENT_ON_CHILD is set if the inode may care */
- if (!(inode->i_fsnotify_mask & FS_EVENT_ON_CHILD))
+ if (!(parent_mask & FS_EVENT_ON_CHILD))
return 0;
/* this inode might care about child events, does it care about the
* specific set of events that can happen on a child? */
- return inode->i_fsnotify_mask & FS_EVENTS_POSS_ON_CHILD;
+ return parent_mask & FS_EVENTS_POSS_ON_CHILD;
}

/*
@@ -582,7 +584,7 @@ static inline void fsnotify_update_flags(struct dentry *dentry)
/*
* Serialisation of setting PARENT_WATCHED on the dentries is provided
* by d_lock. If inotify_inode_watched changes after we have taken
- * d_lock, the following __fsnotify_update_child_dentry_flags call will
+ * d_lock, the following fsnotify_update_children_dentry_flags call will
* find our entry, so it will spin until we complete here, and update
* us with the new state.
*/
--
2.34.1