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Message-ID: <20200422034609.GH3372712@iweiny-DESK2.sc.intel.com>
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
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