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

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

Hi,

On 2026/3/31 22:59, Mateusz Guzik wrote:
> On Tue, Mar 31, 2026 at 05:54:01PM +0800, Gao Xiang wrote:
>> 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.
> 
> 
> Another way to contribute to the problem is to mass open the same file,
> which results in one ref per fd.
> 
> Or to put it differently, sooner or later the dentry refcount will have
> to switch to 64 bits on 64 bit systems.

Yes, but my own basic question on this is that do we
really need 64-bit refcount for each dentry?

  - do we need to cache so many child dentries at the
    same time? some real use case?

  - do we need to cache so many negative dentries for
    a single directory?

  - do we need to really care mass open the same file?

or just find a way to error out blindly when the
refcount is nearly overflowed? together with this
retain_dentry() change to make cached dentries
in the low watermark.

just my .2 cents since currently I don't work on
vfs stuffs.

Thanks,
Gao Xiang