linux-kernel - Re: [PATCH v2 3/3] libnvdimm: don't flush power-fail protected CPU caches

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Date:   Tue, 5 Jun 2018 19:00:14 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Jiang <dave.jiang@...el.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH v2 3/3] libnvdimm: don't flush power-fail protected CPU caches

On Tue, Jun 5, 2018 at 4:58 PM, Ross Zwisler
<ross.zwisler@...ux.intel.com> wrote:
> This commit:
>
> 5fdf8e5ba566 ("libnvdimm: re-enable deep flush for pmem devices via fsync()")
>
> intended to make sure that deep flush was always available even on
> platforms which support a power-fail protected CPU cache.  An unintended
> side effect of this change was that we also lost the ability to skip
> flushing CPU caches on those power-fail protected CPU cache.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Fixes: 5fdf8e5ba566 ("libnvdimm: re-enable deep flush for pmem devices via fsync()")
> ---
>  drivers/dax/super.c   | 14 +++++++++++++-
>  drivers/nvdimm/pmem.c |  2 ++
>  include/linux/dax.h   |  4 ++++
>  3 files changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index c2c46f96b18c..80253c531a9b 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -152,6 +152,8 @@ enum dax_device_flags {
>         DAXDEV_ALIVE,
>         /* gate whether dax_flush() calls the low level flush routine */
>         DAXDEV_WRITE_CACHE,
> +       /* only flush the CPU caches if they are not power fail protected */
> +       DAXDEV_FLUSH_ON_SYNC,

I'm not grokking why we need DAXDEV_FLUSH_ON_SYNC. The power fail
protected status of the cache only determines the default for
DAXDEV_WRITE_CACHE.