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Date:   Fri, 28 Feb 2020 04:52:55 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Ian Kent <raven@themaw.net>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andreas Dilger <adilger@dilger.ca>,
        Waiman Long <longman@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        linux-doc@vger.kernel.org,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Eric Biggers <ebiggers@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Eric Sandeen <sandeen@redhat.com>
Subject: Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries
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On Fri, Feb 28, 2020 at 12:36:09PM +0800, Ian Kent wrote:

> And let's not forget that file systems are the primary
> source of these and not all create them on lookups.
> I may be mistaken, but I think ext4 does not while xfs
> definitely does.

Both ext4 and xfs bloody well *DO* create hashed negative
dentries on lookups.  There is a pathological case when
they are trying to be case-insensitive (and in that situation
we are SOL - if somebody insists upon mounting with
-o make-it-suck, that's what they bloody well get).

Casefondling idiocy aside, negative lookups are hashed.
On all normal filesystems.  Look for d_splice_alias()
getting passed NULL inode - that's where ->lookup()
instances normally create those.