linux-kernel - Re: [PATCH v3 13/14] filesystem-dax: gate calls to dax_flush() on QUEUE_FLAG

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Date:   Sun, 18 Jun 2017 10:45:57 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm@...ts.01.org, Jan Kara <jack@...e.cz>,
        dm-devel@...hat.com, Matthew Wilcox <mawilcox@...rosoft.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Jeff Moyer <jmoyer@...hat.com>, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org,
        Ross Zwisler <ross.zwisler@...ux.intel.com>, hch@....de
Subject: Re: [PATCH v3 13/14] filesystem-dax: gate calls to dax_flush() on
        QUEUE_FLAG_WC

On Fri, Jun 09, 2017 at 01:24:56PM -0700, Dan Williams wrote:
> Some platforms arrange for cpu caches to be flushed on power-fail. On
> those platforms there is no requirement that the kernel track and flush
> potentially dirty cache lines. Given that we still insert entries into
> the radix for locking purposes this patch only disables the cache flush
> loop, not the dirty tracking.
> 
> Userspace can override the default cache setting via the block device
> queue "write_cache" attribute in sysfs.

NAK.  Please stop using the block infrastructure for dax values.  Have
your own flag and sysfs file in the dax infrastructure and only propagate
it to the block layer for the block devices using dax.