Re: [PATCH 08/13] mm/migrate_device: fix ksmd oops on a migrated KSM page

["David Hildenbrand (Arm)" <david@kernel.org>]

On 11/11/26 19:13, Lorenzo Stoakes (ARM) wrote:
> From: Chris Mason <mason@kernel.org>
> 
> After a KSM page is migrated to device private memory, ksmd can oops,
> and KSM then stops until reboot. A process that maps the page can also
> loop in page faults when it next accesses the page.
> 
> The problem arises because KSM doesn't know not to try to look at device
> private memory that does not exist in the direct map.
> 
> Disallow this from happening by explicitly testing for this scenario in
> __migrate_device_pages().
> 
> Fixes: a5430dda8a3a ("mm/migrate: support un-addressable ZONE_DEVICE page in migration")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Chris Mason <mason@kernel.org>
> [ ljs: tweak commit message, comment ]
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>  mm/migrate_device.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 45e571dbfb4c..7ebd02ec303b 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -1205,6 +1205,15 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>  		}
>  
>  
> +		/*
> +		 * KSM doesn't know not to try to read device private pages, so
> +		 * disallow the combination.
> +		 */
> +		if (folio_is_device_private(newfolio) && folio_test_ksm(folio)) {
> +			src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
> +			goto next;

In general, LGTM. I was just wondering if we could reject that earlier. because
apparently we already allocated a destination folio :)

-- 
Cheers,

David