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

[David Hildenbrand <david@redhat.com>]

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