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Subjectshrink_dentry_list() logics change (was Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects)
On Sat, Jun 29, 2019 at 05:08:44AM +0100, Al Viro wrote:
> > The reason we don't hit that problem with regular memory shrinker is
> > this:
> > unregister_shrinker(&s->s_shrink);
> > fs->kill_sb(s);
> > in deactivate_locked_super(). IOW, shrinker for this fs is gone
> > before we get around to shutdown. And so are all normal sources
> > of dentry eviction for that fs.
> >
> > Your earlier variants all suffer the same problem - picking a page
> > shared by dentries from several superblocks can run into trouble
> > if it overlaps with umount of one of those.

PS: the problem is not gone in the next iteration of the patchset in
question. The patch I'm proposing (including dput_to_list() and _ONLY_
compile-tested) follows. Comments?

diff --git a/fs/dcache.c b/fs/dcache.c
index 8136bda27a1f..dfe21a649c96 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -860,6 +860,32 @@ void dput(struct dentry *dentry)
}
EXPORT_SYMBOL(dput);

+static void __dput_to_list(struct dentry *dentry, struct list_head *list)
+__must_hold(&dentry->d_lock)
+{
+ if (dentry->d_flags & DCACHE_SHRINK_LIST) {
+ /* let the owner of the list it's on deal with it */
+ --dentry->d_lockref.count;
+ } else {
+ if (dentry->d_flags & DCACHE_LRU_LIST)
+ d_lru_del(dentry);
+ if (!--dentry->d_lockref.count)
+ d_shrink_add(dentry, list);
+ }
+}
+
+void dput_to_list(struct dentry *dentry, struct list_head *list)
+{
+ rcu_read_lock();
+ if (likely(fast_dput(dentry))) {
+ rcu_read_unlock();
+ return;
+ }
+ rcu_read_unlock();
+ if (!retain_dentry(dentry))
+ __dput_to_list(dentry, list);
+ spin_unlock(&dentry->d_lock);
+}

/* This must be called with d_lock held */
static inline void __dget_dlock(struct dentry *dentry)
@@ -1088,18 +1114,9 @@ static void shrink_dentry_list(struct list_head *list)
rcu_read_unlock();
d_shrink_del(dentry);
parent = dentry->d_parent;
+ if (parent != dentry)
+ __dput_to_list(parent, list);
__dentry_kill(dentry);
- if (parent == dentry)
- continue;
- /*
- * We need to prune ancestors too. This is necessary to prevent
- * quadratic behavior of shrink_dcache_parent(), but is also
- * expected to be beneficial in reducing dentry cache
- * fragmentation.
- */
- dentry = parent;
- while (dentry && !lockref_put_or_lock(&dentry->d_lockref))
- dentry = dentry_kill(dentry);
}
}

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