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

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Date:   Tue, 21 Apr 2020 20:46:09 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.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 Wed, Apr 22, 2020 at 03:34:07AM +0100, Al Viro wrote:
> 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?

Directories never get S_DAX set.  So the eviction only needs to happen on
inodes.  But that can't happen without dentries also dropping their references.

> 2) how much trouble do we get if such inode is *NOT* evicted for, say, several
> days?

No trouble at all.  Users understand that changing the FS_XFLAG_DAX setting
does _not_ immediately result in S_DAX changing.

It is intended that applications requiring a change of mode would flip the
FS_XFLAG_DAX close the file and wait for the eviction (or force it through a
drop cache if they have permission).

Ira