linux-kernel - Re: [PATCH v3 2/2] dax: add a sysfs knob to control memmap_on

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Date:   Tue, 12 Dec 2023 08:56:33 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     "Verma, Vishal L" <vishal.l.verma@...el.com>
Cc:     "david@...hat.com" <david@...hat.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "lizhijian@...itsu.com" <lizhijian@...itsu.com>
Subject: Re: [PATCH v3 2/2] dax: add a sysfs knob to control
 memmap_on_memory behavior

"Verma, Vishal L" <vishal.l.verma@...el.com> writes:

> On Tue, 2023-12-12 at 08:30 +0800, Huang, Ying wrote:
>> Vishal Verma <vishal.l.verma@...el.com> writes:
>>
>> > Add a sysfs knob for dax devices to control the memmap_on_memory setting
>> > if the dax device were to be hotplugged as system memory.
>> >
>> > The default memmap_on_memory setting for dax devices originating via
>> > pmem or hmem is set to 'false' - i.e. no memmap_on_memory semantics, to
>> > preserve legacy behavior. For dax devices via CXL, the default is on.
>> > The sysfs control allows the administrator to override the above
>> > defaults if needed.
>> >
>> > Cc: David Hildenbrand <david@...hat.com>
>> > Cc: Dan Williams <dan.j.williams@...el.com>
>> > Cc: Dave Jiang <dave.jiang@...el.com>
>> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> > Cc: Huang Ying <ying.huang@...el.com>
>> > Tested-by: Li Zhijian <lizhijian@...itsu.com>
>> > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
>> > Reviewed-by: David Hildenbrand <david@...hat.com>
>> > Signed-off-by: Vishal Verma <vishal.l.verma@...el.com>
>> > ---
>> >  drivers/dax/bus.c                       | 47 +++++++++++++++++++++++++++++++++
>> >  Documentation/ABI/testing/sysfs-bus-dax | 17 ++++++++++++
>> >  2 files changed, 64 insertions(+)
>> >
>> > diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
>> > index 1ff1ab5fa105..2871e5188f0d 100644
>> > --- a/drivers/dax/bus.c
>> > +++ b/drivers/dax/bus.c
>> > @@ -1270,6 +1270,52 @@ static ssize_t numa_node_show(struct device *dev,
>> >  }
>> >  static DEVICE_ATTR_RO(numa_node);
>> >
>> > +static ssize_t memmap_on_memory_show(struct device *dev,
>> > +                                    struct device_attribute *attr, char *buf)
>> > +{
>> > +       struct dev_dax *dev_dax = to_dev_dax(dev);
>> > +
>> > +       return sprintf(buf, "%d\n", dev_dax->memmap_on_memory);
>> > +}
>> > +
>> > +static ssize_t memmap_on_memory_store(struct device *dev,
>> > +                                     struct device_attribute *attr,
>> > +                                     const char *buf, size_t len)
>> > +{
>> > +       struct device_driver *drv = dev->driver;
>> > +       struct dev_dax *dev_dax = to_dev_dax(dev);
>> > +       struct dax_region *dax_region = dev_dax->region;
>> > +       struct dax_device_driver *dax_drv = to_dax_drv(drv);
>> > +       ssize_t rc;
>> > +       bool val;
>> > +
>> > +       rc = kstrtobool(buf, &val);
>> > +       if (rc)
>> > +               return rc;
>> > +
>> > +       if (dev_dax->memmap_on_memory == val)
>> > +               return len;
>> > +
>> > +       device_lock(dax_region->dev);
>> > +       if (!dax_region->dev->driver) {
>> > +               device_unlock(dax_region->dev);
>> > +               return -ENXIO;
>> > +       }
>>
>> I think that it should be OK to write to "memmap_on_memory" if no driver
>> is bound to the device.  We just need to avoid to write to it when kmem
>> driver is bound.
>
> Oh this is just a check on the region driver, not for a dax driver
> being bound to the device. It's the same as what things like
> align_store(), size_store() etc. do for dax device reconfiguration.

Sorry, I misunderstood it.

> That said, it might be okay to remove this check, as this operation
> doesn't change any attributes of the dax region (the other interfaces I
> mentioned above can affect regions, so we want to lock the region
> device). If removing the check, we'd drop the region lock acquisition
> as well.

This sounds good to me.

And is it necessary to check driver type with device_lock()?  Can driver
be changed between checking and lock?

--
Best Regards,
Huang, Ying