linux-ext4 - Re: [PATCH V4 07/13] fs: Add locking for a dynamic address space operations state

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Message-ID: <x49fteyh313.fsf@segfault.boston.devel.redhat.com>
Date:   Tue, 25 Feb 2020 14:37:12 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Christoph Hellwig <hch@....de>, jhallida@...hat.com
Cc:     Dave Chinner <david@...morbit.com>, ira.weiny@...el.com,
        linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V4 07/13] fs: Add locking for a dynamic address space operations state

Christoph Hellwig <hch@....de> writes:

> And my point is that if we ensure S_DAX can only be checked if there
> are no blocks on the file, is is fairly easy to provide the same
> guarantee.  And I've not heard any argument that we really need more
> flexibility than that.  In fact I think just being able to change it
> on the parent directory and inheriting the flag might be more than
> plenty, which would lead to a very simple implementation without any
> of the crazy overhead in this series.

I know of one user who had at least mentioned it to me, so I cc'd him.
Jonathan, can you describe your use case for being able to change a
file between dax and non-dax modes?  Or, if I'm misremembering, just
correct me?

Thanks!
Jeff