Re: [PATCH v3 02/11] KVM: guest_memfd: Handle final folio_put() of guest_memfd pages

[Vlastimil Babka <vbabka@suse.cz>]

On 2/20/25 12:25, David Hildenbrand wrote:
> On 11.02.25 13:11, Fuad Tabba wrote:
>> Before transitioning a guest_memfd folio to unshared, thereby
>> disallowing access by the host and allowing the hypervisor to
>> transition its view of the guest page as private, we need to be
>> sure that the host doesn't have any references to the folio.
>> 
>> This patch introduces a new type for guest_memfd folios, which
>> isn't activated in this series but is here as a placeholder and
>> to facilitate the code in the next patch. This will be used in
>> the future to register a callback that informs the guest_memfd
>> subsystem when the last reference is dropped, therefore knowing
>> that the host doesn't have any remaining references.
>> 
>> Signed-off-by: Fuad Tabba <tabba@google.com>
>> ---
> 
> [...]
> 
>>   static const char *page_type_name(unsigned int page_type)
>> diff --git a/mm/swap.c b/mm/swap.c
>> index 47bc1bb919cc..241880a46358 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -38,6 +38,10 @@
>>   #include <linux/local_lock.h>
>>   #include <linux/buffer_head.h>
>>   
>> +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
>> +#include <linux/kvm_host.h>
>> +#endif
>> +
>>   #include "internal.h"
>>   
>>   #define CREATE_TRACE_POINTS
>> @@ -101,6 +105,11 @@ static void free_typed_folio(struct folio *folio)
>>   	case PGTY_hugetlb:
>>   		free_huge_folio(folio);
>>   		return;
>> +#endif
>> +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
>> +	case PGTY_guestmem:
>> +		kvm_gmem_handle_folio_put(folio);
>> +		return;
> 
> Hm, if KVM is built as a module, will that work? Or would we need the 

Good catch, I guess not?

> core-mm guest_memfd shim that would always be compiled into the core and 
> decouple KVM from guest_memfd ("library")?

That could also help avoid exporting the mpol symbols in the NUMA mempolicy
series?