Re: [PATCH 2/3] Add VM_INIT_DEF_MASK and PRCTL_THP_DISABLE

[Oleg Nesterov <oleg@redhat.com>]

On 01/31, Alex Thorlton wrote:
>
> This patch adds a VM_INIT_DEF_MASK,

Perhaps it makes sense to tell a bit more... We add this mask to preserve
VM_NOHUGEPAGE after fork/exec. And this is obviously affects s390, say the
result of KVM_S390_ENABLE_SIE will be preserved.

I hope this is fine, but should be documented and it would be nice to have
the acks from Gerald.


> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1996,6 +1996,23 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  		if (arg2 || arg3 || arg4 || arg5)
>  			return -EINVAL;
>  		return current->no_new_privs ? 1 : 0;
> +	case PR_GET_THP_DISABLE:
> +		if (arg2 || arg3 || arg4 || arg5)
> +			return -EINVAL;

Cosmetic, but PR_GET_THP_DISABLE only needs to check arg2.

OTOH,

> +	case PR_SET_THP_DISABLE:
> +		if (arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		down_write(&me->mm->mmap_sem);
> +		if (option == PR_SET_THP_DISABLE) {
> +			if (arg2)
> +				me->mm->def_flags |= VM_NOHUGEPAGE;
> +			else
> +				me->mm->def_flags &= ~VM_NOHUGEPAGE;
> +		} else {
> +			error = !!(me->mm->def_flags & VM_NOHUGEPAGE);
> +		}
> +		up_write(&me->mm->mmap_sem);
> +		break;

Perhaps _GET_ doesn't even need ->mmap_sem, I do not see how the lockless
"&" can get the inconsistent result. But I am fine either way.

Oleg.

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