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

[Gao Xiang]

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