Re: [PATCH] arm: Do not add padding alignment for hugetlbfs backed memory

[Marc Zyngier <maz@kernel.org>]

On Wed, 05 Apr 2023 12:09:05 +0100,
Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
> 
> The arm code tries to align the memory allocation size to 2M to potentially
> make use of the transparent hugepages. But this would be problematic if we
> try to allocate from the hugetlbfs, where the allocation size could be more than
> 2M. Given we support upto 1G, let use leave it to the user to align the
> requested memory when hugetlbfs is used.
> 
> Without the patch:
>  $ echo 1 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>  $ mount -t hugetlbfs -o pagesize=1G none /root/hugemem/
>  $  lkvm run -m 1024 --hugetlbfs /root/hugemem/  ...
>    # lkvm run -k ...  -m 1024 -c 6
>    Fatal: Can't ftruncate for mem mapping size 1075838976
> 
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
>  arm/kvm.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arm/kvm.c b/arm/kvm.c
> index d51cc15d..9f958232 100644
> --- a/arm/kvm.c
> +++ b/arm/kvm.c
> @@ -37,7 +37,9 @@ void kvm__init_ram(struct kvm *kvm)
>  	 * 2M trumps 64K, so let's go with that.
>  	 */
>  	kvm->ram_size = kvm->cfg.ram_size;
> -	kvm->arch.ram_alloc_size = kvm->ram_size + SZ_2M;
> +	kvm->arch.ram_alloc_size = kvm->ram_size;
> +	if (!kvm->cfg.hugetlbfs_path)
> +		kvm->arch.ram_alloc_size += SZ_2M;
>  	kvm->arch.ram_alloc_start = mmap_anon_or_hugetlbfs(kvm,
>  						kvm->cfg.hugetlbfs_path,
>  						kvm->arch.ram_alloc_size);

Seems sensible. For hugetlbfs, we're guaranteed that the memory is
aligned, so no need for any additional alignment (which is what this
+2MB was about).

Acked-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.