Re: [PATCH v2 2/3] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem

Christian Brauner
2022-10-21T09:18:20Z

On Thu, Oct 20, 2022 at 06:03:09PM -0700, Stephen Brennan wrote:
> When an inode is interested in events on its children, it must set
> DCACHE_FSNOTIFY_PARENT_WATCHED flag on all its children. Currently, when
> the fsnotify connector is removed and i_fsnotify_mask becomes zero, we
> lazily allow __fsnotify_parent() to do this the next time we see an
> event on a child.
>
> However, if the list of children is very long (e.g., in the millions),
> and lots of activity is occurring on the directory, then it's possible
> for many CPUs to end up blocked on the inode spinlock in
> __fsnotify_update_child_flags(). Each CPU will then redundantly iterate
> over the very long list of children. This situation can cause soft
> lockups.
>
> To avoid this, stop lazily updating child flags in __fsnotify_parent().
> Instead, update flags when we disconnect a mark connector. Remember the
> state of the children flags in the fsnotify_mark_connector flags.
> Provide mutual exclusion by holding i_rwsem exclusive while we update
> children, and use the cached state to avoid updating flags
> unnecessarily.
>
> Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
> ---
>
> fs/notify/fsnotify.c | 22 ++++++-
> fs/notify/fsnotify.h | 31 ++++++++-
> fs/notify/mark.c | 106 ++++++++++++++++++++-----------
> include/linux/fsnotify_backend.h | 8 +++
> 4 files changed, 127 insertions(+), 40 deletions(-)
>
> diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
> index 6c338322f0c3..f83eca4fb841 100644
> --- a/fs/notify/fsnotify.c
> +++ b/fs/notify/fsnotify.c
> @@ -103,13 +103,15 @@ 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)
> +bool __fsnotify_update_children_dentry_flags(struct inode *inode)
> {
> struct dentry *alias, *child;
> int watched;
>
> if (!S_ISDIR(inode->i_mode))
> - return;
> + return false;
> +
> + lockdep_assert_held_write(&inode->i_rwsem);
>
> /* determine if the children should tell inode about their events */
> watched = fsnotify_inode_watches_children(inode);
> @@ -133,6 +135,20 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
> spin_unlock(&child->d_lock);
> }
> spin_unlock(&alias->d_lock);
> + return watched;
> +}
> +
> +void __fsnotify_update_child_dentry_flags(struct inode *inode, struct dentry *dentry)
> +{
> + /*
> + * Flag would be cleared soon by
> + * __fsnotify_update_child_dentry_flags(), but as an
> + * optimization, clear it now.
> + */
> + spin_lock(&dentry->d_lock);
> + 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? */
> @@ -203,7 +219,7 @@ 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..182d93014c6b 100644
> --- a/fs/notify/fsnotify.h
> +++ b/fs/notify/fsnotify.h
> @@ -70,11 +70,40 @@ static inline void fsnotify_clear_marks_by_sb(struct super_block *sb)
> fsnotify_destroy_marks(&sb->s_fsnotify_marks);
> }
>
> +static inline bool fsnotify_children_need_update(struct fsnotify_mark_connector *conn,
> + struct inode *inode)
> +{
> + bool watched, flags_set;
> + watched = fsnotify_inode_watches_children(inode);

nit: I'd leave a blank line after the variable declarations. Same for
fsnotify_update_children_dentry_flags() below.

> + flags_set = conn->flags & FSNOTIFY_CONN_FLAG_WATCHES_CHILDREN;
> + return (watched && !flags_set) || (!watched && flags_set);
> +}
> +
> /*
> * 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 bool __fsnotify_update_children_dentry_flags(struct inode *inode);
> +
> +static inline void fsnotify_update_children_dentry_flags(struct fsnotify_mark_connector *conn,
> + struct inode *inode)

Should that be a static inline function in a header seems a bit big. :)