linux-kernel - Re: [PATCH v3 2/4] libnvdimm: unconditionally deep flush on *sync

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Date:   Wed, 6 Jun 2018 10:57:59 -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 v3 2/4] libnvdimm: unconditionally deep flush on *sync

On Wed, Jun 6, 2018 at 9:45 AM, Ross Zwisler
<ross.zwisler@...ux.intel.com> wrote:
> Prior to this commit we would only do a "deep flush" (have nvdimm_flush()
> write to each of the flush hints for a region) in response to an
> msync/fsync/sync call if the nvdimm_has_cache() returned true at the time
> we were setting up the request queue.  This happens due to the write cache
> value passed in to blk_queue_write_cache(), which then causes the block
> layer to send down BIOs with REQ_FUA and REQ_PREFLUSH set.  We do have a
> "write_cache" sysfs entry for namespaces, i.e.:
>
>   /sys/bus/nd/devices/pfn0.1/block/pmem0/dax/write_cache
>
> which can be used to control whether or not the kernel thinks a given
> namespace has a write cache, but this didn't modify the deep flush behavior
> that we set up when the driver was initialized.  Instead, it only modified
> whether or not DAX would flush CPU caches via dax_flush() in response to
> *sync calls.
>
> Simplify this by making the *sync deep flush always happen, regardless of
> the write cache setting of a namespace.  The DAX CPU cache flushing will
> still be controlled the write_cache setting of the namespace.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Suggested-by: Dan Williams <dan.j.williams@...el.com>

Looks, good. I believe we want this one and ["PATCH v3 4/4] libnvdimm:
don't flush power-fail protected CPU caches" marked for -stable and
tagged with:

Fixes: 5fdf8e5ba566 ("libnvdimm: re-enable deep flush for pmem devices
via fsync()")

...any concerns with that?