Re: Negative dentries on Linux SMB filesystems

[Paulo Alcantara <pc@manguebit.com>]

Shyam Prasad N <nspmangalore@gmail.com> writes:

> Hi Paulo,
>
> Thanks for your replies.
>
> On Mon, Jan 13, 2025 at 8:55 PM Paulo Alcantara <pc@manguebit.com> wrote:
>>
>> Shyam Prasad N <nspmangalore@gmail.com> writes:
>>
>> > Ideally, negative dentries should allow a filename lookup to happen
>> > entirely from the dentry cache if the lookup had happened once
>> > already. But I noticed that the SMB client goes to the server every
>> > time we do a stat of a file that does not exist.
>>
>> This is a network filesystem.  If the last lookup ended up with a
>> negative dentry in dcache, that doesn't mean the file won't exist the
>> next time we look it up again.  The file could have been created by a
>> different client, so we need to query it on server.
>
> I agree. But we do have tools to trade performance for accuracy using
> parameters like actimeo/acdirmax/acregmax.

Do you mean using these parameters for negative dentries?  These are
used for caching file attributes of files and directories, which means
they are all positive dentries.

> So we can avoid going to the server each time if it's within some interval.
> If the server gives us dir leases, we can be sure that the dentries
> have not changed without us knowing. So we can definitely cache the
> negative dentries till as long as we have the lease.

Yes, that could be done with directory leases.

Note that negative dentries are also cached when @lookupCacheEnabled is
set.