Re: bad things when too many negative dentries in a directory

[Ian Kent <raven@themaw.net>]

On 11/4/25 23:40, Miklos Szeredi wrote:
> On Fri, 11 Apr 2025 at 16:47, Christian Brauner <brauner@kernel.org> wrote:
>
>> Note that we have a new sysctl:
>>
>> /proc/sys/fs/dentry-negative
>>
>> that can be used to control the negative dentry policy because any
>> generic change that we tried to make has always resulted in unacceptable
>> regressions for someone's workload. Currently we only allow it to be set
>> to 1 (default 0). If set to 1 it will not create negative dentries
>> during unlink. If that's sufficient than recommend this to users that
>> suffer from this problem if not consider adding another sensitive
>> policy.
> Okay, I'll forward that info.
>
> However, hundreds of millions of negative dentries can be created
> rather efficiently without unlink, though this one probably doesn't
> happen under normal circumstances.  Allowing this to starve the
> scheduler for an arbitrary long time is not a good idea in any case,
> so the fsnotify problem needs some other solution, and I suspect that
> it's not to disable negative caching completely, as that would be a
> major bummer.

I know that the most recent case we have seen of this would probably

be resolved by the sysctl but this was not the first recent case we had.

Unfortunately I can't remember the details, all I remember is it was

similar but not quite the same.


In any case it's quite possible that many files can be processed, opened

and then closed, not unlinked.


So I think it's worth considering this as well.

>
> But the idea of leaving negative dentries off d_children is
> independent of caching policy.  The lookup cache would work fine
> without d_sib being chained, it only needs careful thought in
>
> 1) putting the dentry on d_children when it's turned into positive
> 2) getting the dentry off d_children when it's turned into negative.

That shouldn't be too difficult to do ... sounds like a good idea to me.


Ian