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Date:   Wed, 3 Apr 2019 02:44:14 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     NeilBrown <neilb@suse.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        "Tobin C. Harding" <tobin@kernel.org>,
        Mauro Carvalho Chehab <mchehab@s-opensource.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 00/24] Convert vfs.txt to vfs.rst
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On Wed, Apr 03, 2019 at 12:00:54PM +1100, NeilBrown wrote:

> I would find it useful if the documentation said something about why
> this API exists at all.  As you say, it cannot change the dentry - so
> what is it expected to do.

*shrug*

It tell filesystem that this dentry is going away now.

> I had a look at the two in-tree users and my guess is that it can be
> useful if the filesystem caches some other information which would be
> invalidated by a dentry being removed.
> I *think* cephfs has a flag which records if "All entries in a directory
> are currently in the dcache".  When a dentry is pruned, that flag needs
> to be cleared.
> 
> i.e. ->d_prune allows a filesystem to maintain summary state about what
>    is currently in the dcache.

For one thing...  Or, if you keep a (non-counting) reference to that
dentry, this tells you to forget it, etc.

I can think of other uses; it's really just telling the fs that this
is becoming an ex-parrot.