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

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi,

On 2026/3/31 17:39, Christian Brauner wrote:
> 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.

JFYI, another issue we once observed on user workloads is that

`d_lockref.count` can exceed `int` on very very large
directories in reality (also combined with cached
negative dentries).

It can be a real overflow, this commit can help but it
doesn't strictly resolve this, anyway.

Thanks,
Gao Xiang