Re: [PATCH v2] hv_balloon: Fallback to generic_online_page() for non-HV hot added mem

[Wei Liu]

On Tue, Jan 07, 2025 at 10:09:18AM -0800, Jacob Pan wrote:
> The Hyper-V balloon driver installs a custom callback for handling page
> onlining operations performed by the memory hotplug subsystem. This
> custom callback is global, and overrides the default callback
> (generic_online_page) that Linux otherwise uses. The custom callback
> properly handles memory that is hot-added by the balloon driver as part
> of a Hyper-V hot-add region.
> 
> But memory can also be hot-added directly by a device driver for a vPCI
> device, particularly GPUs. In such a case, the custom callback installed by
> the balloon driver runs, but won't find the page in its hot-add region list
> and doesn't online it, which could cause driver initialization failures.
> 
> Fix this by having the balloon custom callback run generic_online_page()
> when the page isn't part of a Hyper-V hot-add region, thereby doing the
> default Linux behavior. This allows device driver hot-adds to work
> properly. Similar cases are handled the same way in the virtio-mem driver.
> 
> Suggested-by: Vikram Sethi <vsethi@xxxxxxxxxx>
> Tested-by: Michael Frohlich <mfrohlich@xxxxxxxxxxxxx>
> Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Signed-off-by: Jacob Pan <jacob.pan@xxxxxxxxxxxxxxxxxxx>

Applied to hyperv-next. Thanks!