Re: Negative dentries on Linux SMB filesystems

[Paulo Alcantara <pc@manguebit.com>]

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 investigated this more and it looks like vfs_getattr does make use
> of negative dentry, but the revalidate always comes to
> cifs_d_revalidate even for negative dentries. And we do not have the
> code necessary to deal with it.

I think we do.  Check the places where we return 0 from
cifs_d_revalidate(), meaning that the dentry will need to be looked up
again and hopefully instantiated (e.g. file was created on server).

> We do use d_really_is_positive before we do the dentry validation, but
> it looks like that comes to us as success, even in case of
> non-existent dentries. Is this expected?

I don't think so.

Al, am I missing someting?