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

[Chenyi Qiang <chenyi.qiang@intel.com>]

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.

>