Re: [PATCH v3 0/3] fsnotify: fix softlockups iterating over d_subdirs

Amir Goldstein
2022-11-02T08:55:25Z

[-- Attachment #1: Type: text/plain, Size: 5391 bytes --]

On Tue, Nov 1, 2022 at 10:49 PM Stephen Brennan
<stephen.s.brennan@oracle.com> wrote:
>
> Hi Jan,
>
> Jan Kara <jack@suse.cz> writes:
> > Hi Stephen!
> >
> > On Thu 27-10-22 17:10:13, Stephen Brennan wrote:
> >> Here is v3 of the patch series. I've taken all of the feedback,
> >> thanks Amir, Christian, Hilf, et al. Differences are noted in each
> >> patch.
> >>
> >> I caught an obvious and silly dentry reference leak: d_find_any_alias()
> >> returns a reference, which I never called dput() on. With that change, I
> >> no longer see the rpc_pipefs issue, but I do think I need more testing
> >> and thinking through the third patch. Al, I'd love your feedback on that
> >> one especially.
> >>
> >> Thanks,
> >> Stephen
> >>
> >> Stephen Brennan (3):
> >> fsnotify: Use d_find_any_alias to get dentry associated with inode
> >> fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem
> >> fsnotify: allow sleepable child flag update
> >
> > Thanks for the patches Stephen and I'm sorry for replying somewhat late.
>
> Absolutely no worries, these things take time. Thanks for taking a look!
>
> > The first patch is a nobrainer. The other two patches ... complicate things
> > somewhat more complicated than I'd like. I guess I can live with them if we
> > don't find a better solution but I'd like to discuss a bit more about
> > alternatives.
>
> Understood!
>
> > So what would happen if we just clear DCACHE_FSNOTIFY_PARENT_WATCHED in
> > __fsnotify_parent() for the dentry which triggered the event and does not
> > have watched parent anymore and never bother with full children walk? I
> > suppose your contention problems will be gone, we'll just pay the price of
> > dget_parent() + fsnotify_inode_watches_children() for each child that
> > falsely triggers instead of for only one. Maybe that's not too bad? After
> > all any event upto this moment triggered this overhead as well...
>
> This is an interesting idea. It came across my mind but I don't think I
> considered it seriously because I assumed that it was too big a change.
> But I suppose in the process I created an even bigger change :P
>
> The false positive dget_parent() + fsnotify_inode_watches_children()
> shouldn't be too bad. I could see a situation where there's a lot of
> random accesses within a directory, where the dget_parent() could cause
> some contention over the parent dentry. But to be fair, the performance
> would have been the same or worse while fsnotify was active in that
> case, and the contention would go away as most of the dentries get their
> flags cleared. So I don't think this is a problem.
>

I also don't think that is a problem.

> > Am I missing something?
>
> I think there's one thing missed here. I understand you'd like to get
> rid of the extra flag in the connector. But the advantage of the flag is
> avoiding duplicate work by saving a bit of state. Suppose that a mark is
> added to a connector, which causes fsnotify_inode_watches_children() to
> become true. Then, any subsequent call to fsnotify_recalc_mask() must
> call __fsnotify_update_child_dentry_flags(), even though the child
> dentry flags don't need to be updated: they're already set. For (very)
> large directories, this can take a few seconds, which means that we're
> doing a few extra seconds of work each time a new mark is added to or
> removed from a connector in that case. I can't imagine that's a super
> common workload though, and I don't know if my customers do that (my
> guess would be no).
>
> For an example of a test workload where this would make a difference:
> one of my test cases is to create a directory with millions of negative
> dentries, and then run "for i in {1..20}; do inotifywait $DIR & done".
> With the series as-is, only the first task needs to do the child flag
> update. With your proposed alternative, each task would re-do the
> update.
>
> If we were to manage to get __fsnotify_update_child_dentry_flags() to
> work correctly, and safely, with the ability to drop the d_lock and
> sleep, then I think that wouldn't be too much of a problem, because then
> other spinlock users of the directory will get a chance to grab it, and
> so we don't risk softlockups. Without the sleepable iterations, it would
> be marginally worse than the current solution, but I can't really
> comment _how_ much worse because like I said, it doesn't sound like a
> frequent usage pattern.
>
> I think I have a _slight_ preference for the current design, but I see
> the appeal of simpler code, and your design would still improve things a
> lot! Maybe Amir has an opinion too, but of course I'll defer to what you
> want.
>

IIUC, patches #1+#3 fix a reproducible softlock, so if Al approves them,
I see no reason to withhold.

With patches #1+#3 applied, the penalty is restricted to adding or
removing/destroying multiple marks on very large dirs or dirs with
many negative dentries.

I think that fixing the synthetic test case of multiple added marks
is rather easy even without DCACHE_FSNOTIFY_PARENT_WATCHED.
Something like the attached patch is what Jan had suggested in the
first place.

The synthetic test case of concurrent add/remove watch on the same
dir may still result in multiple children iterations, but that will usually
not be avoided even with DCACHE_FSNOTIFY_PARENT_WATCHED
and probably not worth optimizing for.

Thoughts?

Thanks,
Amir.

[-- Attachment #2: fsnotify-clear-PARENT_WATCHED-flags-lazily.patch --]
[-- Type: text/x-patch, Size: 6791 bytes --]

From c8ea1d84397c26ce334bff9d9f721400cebb88dd Mon Sep 17 00:00:00 2001
From: Amir Goldstein <amir73il@gmail.com>
Date: Wed, 2 Nov 2022 10:28:01 +0200
Subject: [PATCH] fsnotify: clear PARENT_WATCHED flags lazily

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>
---
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 1e541a9bd12b..f60078d6bb97 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 fcc68b8a40fd..614bce0e7261 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 a31423c376a7..bd90bcf6c3b0 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -586,12 +586,14 @@ 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;
}

/*
@@ -605,7 +607,7 @@ 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.
*/
--
2.25.1