[PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend

[PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend

[Chenyi Qiang]

Currently, private memory and shared memory have different backend in
CoCo VMs. It is possible for users to specify the shared memory with
hugetlbfs backend while private memory with guest_memfd backend only
supports 4K page size. In this case, ram_block->page_size is different
from the host page size which will trigger the assertion when getting
block size. Relax the restriction to allow shared memory to use
hugetlbfs backend.

Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage RAMBlock with guest_memfd")
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
---
 system/ram-block-attributes.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c
index 68e8a027032..0f39ccf9090 100644
--- a/system/ram-block-attributes.c
+++ b/system/ram-block-attributes.c
@@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const RamBlockAttributes *attr)
      * Because page conversion could be manipulated in the size of at least 4K
      * or 4K aligned, Use the host page size as the granularity to track the
      * memory attribute.
+     * When hugetlbfs is used as backend of shared memory, ram_block->page_size
+     * is different from host page size. So it is not appropriate to use
+     * ram_block->page_size here.
      */
-    g_assert(attr && attr->ram_block);
-    g_assert(attr->ram_block->page_size == qemu_real_host_page_size());
-    return attr->ram_block->page_size;
+    return qemu_real_host_page_size();
 }
 
 
-- 
2.43.5

Re: [PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend

[Peter Xu]

On Fri, Oct 17, 2025 at 04:14:23PM +0800, Chenyi Qiang wrote:
> Currently, private memory and shared memory have different backend in
> CoCo VMs. It is possible for users to specify the shared memory with
> hugetlbfs backend while private memory with guest_memfd backend only
> supports 4K page size. In this case, ram_block->page_size is different
> from the host page size which will trigger the assertion when getting
> block size. Relax the restriction to allow shared memory to use
> hugetlbfs backend.

Is it proper I add this into the commit message too, when queue it?

  This patch fixes a regression of using hugetlbfs memory backends with
  CoCo VMs, with or without VFIO devices' presence.

Thanks,

> 
> Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage RAMBlock with guest_memfd")
> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
> ---
>  system/ram-block-attributes.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c
> index 68e8a027032..0f39ccf9090 100644
> --- a/system/ram-block-attributes.c
> +++ b/system/ram-block-attributes.c
> @@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const RamBlockAttributes *attr)
>       * Because page conversion could be manipulated in the size of at least 4K
>       * or 4K aligned, Use the host page size as the granularity to track the
>       * memory attribute.
> +     * When hugetlbfs is used as backend of shared memory, ram_block->page_size
> +     * is different from host page size. So it is not appropriate to use
> +     * ram_block->page_size here.
>       */
> -    g_assert(attr && attr->ram_block);
> -    g_assert(attr->ram_block->page_size == qemu_real_host_page_size());
> -    return attr->ram_block->page_size;
> +    return qemu_real_host_page_size();
>  }
>  
>  
> -- 
> 2.43.5
> 

-- 
Peter Xu

Re: [PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend

[David Hildenbrand]

On 17.10.25 10:14, Chenyi Qiang wrote:
> Currently, private memory and shared memory have different backend in
> CoCo VMs. It is possible for users to specify the shared memory with
> hugetlbfs backend while private memory with guest_memfd backend only
> supports 4K page size. In this case, ram_block->page_size is different
> from the host page size which will trigger the assertion when getting
> block size. Relax the restriction to allow shared memory to use
> hugetlbfs backend.
> 
> Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage RAMBlock with guest_memfd")
> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
> ---
>   system/ram-block-attributes.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c
> index 68e8a027032..0f39ccf9090 100644
> --- a/system/ram-block-attributes.c
> +++ b/system/ram-block-attributes.c
> @@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const RamBlockAttributes *attr)
>        * Because page conversion could be manipulated in the size of at least 4K
>        * or 4K aligned, Use the host page size as the granularity to track the
>        * memory attribute.
> +     * When hugetlbfs is used as backend of shared memory, ram_block->page_size
> +     * is different from host page size. So it is not appropriate to use
> +     * ram_block->page_size here.

But are we sure everything else is working as expected and that this is 
not a check that prevents other code from doing the wrong thing?

I recall that punching holes was problematic as the VM shares/unshared 
4k chunks.

-- 
Cheers

David / dhildenb

Re: [PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend

[Chenyi Qiang]

On 10/17/2025 11:13 PM, David Hildenbrand wrote:
> On 17.10.25 10:14, Chenyi Qiang wrote:
>> Currently, private memory and shared memory have different backend in
>> CoCo VMs. It is possible for users to specify the shared memory with
>> hugetlbfs backend while private memory with guest_memfd backend only
>> supports 4K page size. In this case, ram_block->page_size is different
>> from the host page size which will trigger the assertion when getting
>> block size. Relax the restriction to allow shared memory to use
>> hugetlbfs backend.
>>
>> Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage RAMBlock with guest_memfd")
>> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
>> ---
>>   system/ram-block-attributes.c | 7 ++++---
>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c
>> index 68e8a027032..0f39ccf9090 100644
>> --- a/system/ram-block-attributes.c
>> +++ b/system/ram-block-attributes.c
>> @@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const RamBlockAttributes *attr)
>>        * Because page conversion could be manipulated in the size of at least 4K
>>        * or 4K aligned, Use the host page size as the granularity to track the
>>        * memory attribute.
>> +     * When hugetlbfs is used as backend of shared memory, ram_block->page_size
>> +     * is different from host page size. So it is not appropriate to use
>> +     * ram_block->page_size here.
> 
> But are we sure everything else is working as expected and that this is not a check that prevents other code from doing the wrong thing?

I think so. The block size must be 4K due to the page conversion could be in the size of 4K and we use "bitmap" to track the status.
I originally missed the case of hugetlb so added an assert() here. But it is allowed to use hugetlb as shared memory backend
before shared device assignment patches were introduced.

> 
> I recall that punching holes was problematic as the VM shares/unshared 4k chunks.

I can see the kvm_convert_memory() will skip ram_block_discard_range() if using hugetlb backend.
It will cause the double-memory consumption (*). Any other problem?

[*] https://lore.kernel.org/qemu-devel/5f65ec5d-7079-4b32-8e40-b9ece3bdd47c@redhat.com/

> 

Re: [PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend

[David Hildenbrand]

On 20.10.25 12:32, Chenyi Qiang wrote:
> 
> 
> On 10/17/2025 11:13 PM, David Hildenbrand wrote:
>> On 17.10.25 10:14, Chenyi Qiang wrote:
>>> Currently, private memory and shared memory have different backend in
>>> CoCo VMs. It is possible for users to specify the shared memory with
>>> hugetlbfs backend while private memory with guest_memfd backend only
>>> supports 4K page size. In this case, ram_block->page_size is different
>>> from the host page size which will trigger the assertion when getting
>>> block size. Relax the restriction to allow shared memory to use
>>> hugetlbfs backend.
>>>
>>> Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage RAMBlock with guest_memfd")
>>> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
>>> ---
>>>    system/ram-block-attributes.c | 7 ++++---
>>>    1 file changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c
>>> index 68e8a027032..0f39ccf9090 100644
>>> --- a/system/ram-block-attributes.c
>>> +++ b/system/ram-block-attributes.c
>>> @@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const RamBlockAttributes *attr)
>>>         * Because page conversion could be manipulated in the size of at least 4K
>>>         * or 4K aligned, Use the host page size as the granularity to track the
>>>         * memory attribute.
>>> +     * When hugetlbfs is used as backend of shared memory, ram_block->page_size
>>> +     * is different from host page size. So it is not appropriate to use
>>> +     * ram_block->page_size here.
>>
>> But are we sure everything else is working as expected and that this is not a check that prevents other code from doing the wrong thing?
> 
> I think so. The block size must be 4K due to the page conversion could be in the size of 4K and we use "bitmap" to track the status.

Indeed.

> I originally missed the case of hugetlb so added an assert() here. But it is allowed to use hugetlb as shared memory backend
> before shared device assignment patches were introduced.
> 
>>
>> I recall that punching holes was problematic as the VM shares/unshared 4k chunks.
> 
> I can see the kvm_convert_memory() will skip ram_block_discard_range() if using hugetlb backend.
> It will cause the double-memory consumption (*). Any other problem?

Right.


What we should be doing is unifying the retrieval of the block size in 
ram_block_attributes_create() as well. That's where we allocate it.

So either

a) Use qemu_real_host_page_size() everywhere.

b) Use ram_block_attributes_get_block_size() everywhere.

Could be done in a separate patch.

-- 
Cheers

David / dhildenb

Re: [PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend

[David Hildenbrand]

On 17.10.25 10:14, Chenyi Qiang wrote:
> Currently, private memory and shared memory have different backend in
> CoCo VMs. It is possible for users to specify the shared memory with
> hugetlbfs backend while private memory with guest_memfd backend only
> supports 4K page size. In this case, ram_block->page_size is different
> from the host page size which will trigger the assertion when getting
> block size. Relax the restriction to allow shared memory to use
> hugetlbfs backend.

Please mention that the VM can do conversion in base page granularity, 
consequently that is the granularity in which we have to track things.

And in the relevant setups we assume that target page size == host page 
size.

> 
> Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage RAMBlock with guest_memfd")
> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
> ---
>   system/ram-block-attributes.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c
> index 68e8a027032..0f39ccf9090 100644
> --- a/system/ram-block-attributes.c
> +++ b/system/ram-block-attributes.c
> @@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const RamBlockAttributes *attr)
>        * Because page conversion could be manipulated in the size of at least 4K
>        * or 4K aligned, Use the host page size as the granularity to track the
>        * memory attribute.
> +     * When hugetlbfs is used as backend of shared memory, ram_block->page_size
> +     * is different from host page size. So it is not appropriate to use
> +     * ram_block->page_size here.

I don't think that comment is required. The comment above already states 
the relevant thing: "Use the host page size as the granularity to track 
the memory attribute."

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb

Re: [PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend

[Chenyi Qiang]

On 10/20/2025 7:31 PM, David Hildenbrand wrote:
> On 20.10.25 12:32, Chenyi Qiang wrote:
>>
>>
>> On 10/17/2025 11:13 PM, David Hildenbrand wrote:
>>> On 17.10.25 10:14, Chenyi Qiang wrote:
>>>> Currently, private memory and shared memory have different backend in
>>>> CoCo VMs. It is possible for users to specify the shared memory with
>>>> hugetlbfs backend while private memory with guest_memfd backend only
>>>> supports 4K page size. In this case, ram_block->page_size is different
>>>> from the host page size which will trigger the assertion when getting
>>>> block size. Relax the restriction to allow shared memory to use
>>>> hugetlbfs backend.
>>>>
>>>> Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage RAMBlock with guest_memfd")
>>>> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
>>>> ---
>>>>    system/ram-block-attributes.c | 7 ++++---
>>>>    1 file changed, 4 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c
>>>> index 68e8a027032..0f39ccf9090 100644
>>>> --- a/system/ram-block-attributes.c
>>>> +++ b/system/ram-block-attributes.c
>>>> @@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const RamBlockAttributes *attr)
>>>>         * Because page conversion could be manipulated in the size of at least 4K
>>>>         * or 4K aligned, Use the host page size as the granularity to track the
>>>>         * memory attribute.
>>>> +     * When hugetlbfs is used as backend of shared memory, ram_block->page_size
>>>> +     * is different from host page size. So it is not appropriate to use
>>>> +     * ram_block->page_size here.
>>>
>>> But are we sure everything else is working as expected and that this is not a check that prevents other code from doing the wrong thing?
>>
>> I think so. The block size must be 4K due to the page conversion could be in the size of 4K and we use "bitmap" to track the status.
> 
> Indeed.
> 
>> I originally missed the case of hugetlb so added an assert() here. But it is allowed to use hugetlb as shared memory backend
>> before shared device assignment patches were introduced.
>>
>>>
>>> I recall that punching holes was problematic as the VM shares/unshared 4k chunks.
>>
>> I can see the kvm_convert_memory() will skip ram_block_discard_range() if using hugetlb backend.
>> It will cause the double-memory consumption (*). Any other problem?
> 
> Right.
> 
> 
> What we should be doing is unifying the retrieval of the block size in ram_block_attributes_create() as well. That's where we allocate it.
> 
> So either
> 
> a) Use qemu_real_host_page_size() everywhere.
> 
> b) Use ram_block_attributes_get_block_size() everywhere.
> 
> Could be done in a separate patch.

Agree. Will clean up it and modify the commit message mentioned by you and Peter in other mail threads.

>