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

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Message-ID: <CAPcyv4hwAseE1NR75Zs9VMM3jiYszLFn9w6z9tzMYOFA4Dgpmw@mail.gmail.com>
Date:   Mon, 24 Apr 2017 10:07:12 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linda Knippers <linda.knippers@....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 10:03 AM, Linda Knippers <linda.knippers@....com> wrote:
> On 04/21/2017 07:48 PM, Dan Williams wrote:
>> 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.
>
> Why would a user need to flush a disabled namespace?

For an application that wants to shutdown and sync. Basically I wanted
to make it clear that with this interface the buffers can be synced
regardless of any downstream namespace configuration or state.

>
>> 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.
>
> It seems a little odd to me that reading a read-only attribute both
> tells you that the device has flush hints and also triggers a flush.
> This means that anyone at any time can cause a flush.  Do we want that?

No, I'm making the change that Masayoshi-san suggested to move the
flush to a write operation... assuming we move forward given Jeff's
concern.