linux-kernel - Re: [PATCH] libnvdimm, region: sysfs trigger for nvdimm

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Date:   Mon, 24 Apr 2017 09:36:47 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] libnvdimm, region: sysfs trigger for nvdimm_flush()

On Mon, Apr 24, 2017 at 9:26 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Dan Williams <dan.j.williams@...el.com> writes:
>
>> The nvdimm_flush() mechanism helps to reduce the impact of an ADR
>> (asynchronous-dimm-refresh) failure. The ADR mechanism handles flushing
>> platform WPQ (write-pending-queue) buffers when power is removed. The
>> nvdimm_flush() mechanism performs that same function on-demand.
>>
>> When a pmem namespace is associated with a block device, an
>> nvdimm_flush() is triggered with every block-layer REQ_FUA, or REQ_FLUSH
>> request. However, when a namespace is in device-dax mode, or namespaces
>> are disabled, userspace needs another path.
>>
>> The new 'flush' attribute is visible when it can be determined that the
>> interleave-set either does, or does not have DIMMs that expose WPQ-flush
>> addresses, "flush-hints" in ACPI NFIT terminology. It returns "1" and
>> flushes DIMMs, or returns "0" the flush operation is a platform nop.
>>
>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>
> NACK.  This should function the same way it does for a pmem device.
> Wire up sync.

We don't have dirty page tracking for device-dax, without that I don't
think we should wire up the current sync calls. I do think we need a
more sophisticated sync syscall interface eventually that can select
which level of flushing is being performed (page cache vs cpu cache vs
platform-write-buffers). Until then I think this sideband interface
makes sense and sysfs is more usable than an ioctl.