linux-ext4 - Re: [PATCH V9 09/11] fs: Introduce DCACHE

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Date:   Wed, 22 Apr 2020 03:34:07 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     ira.weiny@...el.com, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V9 09/11] fs: Introduce DCACHE_DONTCACHE

On Tue, Apr 21, 2020 at 01:25:19PM -0700, Darrick J. Wong wrote:

> > DCACHE_DONTCACHE indicates a dentry should not be cached on final
> > dput().
> > 
> > Also add a helper function to mark DCACHE_DONTCACHE on all dentries
> > pointing to a specific inode when that inode is being set I_DONTCACHE.
> > 
> > This facilitates dropping dentry references to inodes sooner which
> > require eviction to swap S_DAX mode.

Explain, please.  Questions:

1) does that ever happen to directories?
2) how much trouble do we get if such inode is *NOT* evicted for, say, several
days?