linux-kernel - Re: [PATCH V6 6/8] fs/xfs: Combine xfs_diflags_to_linux() and xfs_diflags_to

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Date:   Wed, 8 Apr 2020 17:22:03 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V6 6/8] fs/xfs: Combine xfs_diflags_to_linux() and
 xfs_diflags_to_iflags()

On Thu, Apr 09, 2020 at 09:58:36AM +1000, Dave Chinner wrote:
> On Wed, Apr 08, 2020 at 02:28:30PM -0700, Dan Williams wrote:
> > On Wed, Apr 8, 2020 at 2:02 PM Dave Chinner <david@...morbit.com> wrote:
> > > THis leads to an obvious conclusion: if we never clear the in memory
> > > S_DAX flag, we can actually clear the on-disk flag safely, so that
> > > next time the inode cycles into memory it won't be using DAX. IOWs,
> > > admins can stop the applications, clear the DAX flag and drop
> > > caches. This should result in the inode being recycled and when the
> > > app is restarted it will run without DAX. No ned for deleting files,
> > > copying large data sets, etc just to turn off an inode flag.
> > 
> > Makes sense, but is that sufficient? I recall you saying there might
> > be a multitude of other reasons that the inode is not evicted, not the
> > least of which is races [1]. Does this need another flag, lets call it
> > "dax toggle" to track the "I requested the inode to clear the flag,
> > but on cache-flush + restart the inode never got evicted" case.
> 
> You mean something like XFS_IDONTCACHE?
> 
> i.e. the functionality already exists in XFS to selectively evict an
> inode from cache when the last reference to it is dropped rather
> than let it go to the LRUs and hang around in memory.
> 
> That flag can be set when changing the on disk DAX flag, and we can
> tweak how it works so new cache hits don't clear it (as happens
> now). Hence the only thing that can prevent eviction are active
> references.
> 
> That means we'll still need to stop the application and drop_caches,
> because we need to close all the files and purge the dentries that
> hold references to the inode before it can be evicted.

That sounds like a great idea...

Jan?  Christoph?

I will reiterate though that any delayed S_DAX change be done as a follow on
series to the one proposed here.

Ira