Re: [RFC PATCH] vfs: limit directory child dentry retention

[Christian Brauner <brauner@kernel.org>]

On Tue, Mar 31, 2026 at 09:29:09AM +0800, Ian Kent wrote:
> If there's a very large number of children present in a directory dentry
> then the benifit from retaining stale child dentries for re-use can
> become ineffective. Even hashed lookup can become ineffective as hash
> chains grow, time taken to umount a file system can increase a lot, as
> well as child dentry traversals resulting in lock held too long log
> messages.

Fwiw, there's also e6957c99dca5 ("vfs: Add a sysctl for automated deletion of dentry")

This patch introduces the concept conditionally, where the associated
dentry is deleted only when the user explicitly opts for it during file
removal. A new sysctl fs.automated_deletion_of_dentry is added for this
purpose. Its default value is set to 0.

I have no massive objections to your approach. It feels a bit hacky tbh
as it seems to degrade performance for new workloads in favor old
workloads. The LRU should sort this out though.

> But when a directory dentry has a very large number of children the
> parent dentry reference count is dominated by the contribution of its
> children. So it makes sense to not retain dentries if the parent
> reference count is large.
> 
> Setting some large high water mark (eg. 500000) over which dentries
> are discarded instead of retained on final dput() would help a lot
> by preventing dentry caching contributing to the problem.
> 
> Signed-off-by: Ian Kent <raven@themaw.net>
> ---
>  Documentation/admin-guide/sysctl/fs.rst |  7 +++++++
>  fs/dcache.c                             | 28 +++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
> index 9b7f65c3efd8..7649254f2d0d 100644
> --- a/Documentation/admin-guide/sysctl/fs.rst
> +++ b/Documentation/admin-guide/sysctl/fs.rst
> @@ -75,6 +75,13 @@ negative dentries which do not map to any files. Instead,
>  they help speeding up rejection of non-existing files provided
>  by the users.
>  
> +dir-stale-max
> +-------------
> +
> +Used to limit the number of stale child dentries retained in a
> +directory before the benifit of caching the dentry is negated by
> +the cost of traversing hash buckets during lookups or enumerating
> +the directory children. Initially set to 500000.
>  
>  file-max & file-nr
>  ------------------
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 7ba1801d8132..298b4c3b1493 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -86,6 +86,14 @@ __cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);
>  
>  EXPORT_SYMBOL(rename_lock);
>  
> +static long dsm_zero = 0;
> +static long dsm_max = ULONG_MAX/2;
> +
> +/* Highwater mark for number of stale entries in a directory (loosely
> + * measured by parent dentry reference count).
> + */
> +static unsigned long dir_stale_max __read_mostly = 500000;
> +
>  static struct kmem_cache *__dentry_cache __ro_after_init;
>  #define dentry_cache runtime_const_ptr(__dentry_cache)
>  
> @@ -216,6 +224,15 @@ static const struct ctl_table fs_dcache_sysctls[] = {
>  		.extra1		= SYSCTL_ZERO,
>  		.extra2		= SYSCTL_ONE,
>  	},
> +	{
> +		.procname	= "dir-stale-max",
> +		.data		= &dir_stale_max,
> +		.maxlen		= sizeof(dir_stale_max),
> +		.mode		= 0644,
> +		.proc_handler	= proc_doulongvec_minmax,
> +		.extra1		= &dsm_zero,
> +		.extra2		= &dsm_max,
> +	},
>  };
>  
>  static const struct ctl_table vm_dcache_sysctls[] = {
> @@ -768,6 +785,17 @@ static inline bool retain_dentry(struct dentry *dentry, bool locked)
>  	if (unlikely(d_flags & DCACHE_DONTCACHE))
>  		return false;
>  
> +	if (dir_stale_max) {
> +		unsigned long p_count;
> +
> +		// If the parent reference count is higher than some large value
> +		// its dominated by the contribution of its children so there's
> +		// no benefit caching the dentry over re-allocating it.
> +		p_count = READ_ONCE(dentry->d_parent->d_lockref.count);
> +		if (unlikely(p_count > dir_stale_max))
> +			return false;
> +	}
> +
>  	// At this point it looks like we ought to keep it.  We also might
>  	// need to do something - put it on LRU if it wasn't there already
>  	// and mark it referenced if it was on LRU, but not marked yet.
> -- 
> 2.53.0
>