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    SubjectRe: [PATCH v3 2/2] dax: add a sysfs knob to control memmap_on_memory behavior
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    On Tue, 2023-12-12 at 08:30 +0800, Huang, Ying wrote:
    > Vishal Verma <vishal.l.verma@intel.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@redhat.com>
    > > Cc: Dan Williams <dan.j.williams@intel.com>
    > > Cc: Dave Jiang <dave.jiang@intel.com>
    > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
    > > Cc: Huang Ying <ying.huang@intel.com>
    > > Tested-by: Li Zhijian <lizhijian@fujitsu.com>
    > > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
    > > Reviewed-by: David Hildenbrand <david@redhat.com>
    > > Signed-off-by: Vishal Verma <vishal.l.verma@intel.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.

    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.

    Dan, any thoughts on this?


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