Re: [PATCH v6 1/3] mm: add new api to enable ksm per process

[Matthew Wilcox <willy@infradead.org>]

On Tue, Apr 11, 2023 at 08:16:46PM -0700, Stefan Roesch wrote:
>  	case PR_SET_VMA:
>  		error = prctl_set_vma(arg2, arg3, arg4, arg5);
>  		break;
> +#ifdef CONFIG_KSM
> +	case PR_SET_MEMORY_MERGE:
> +		if (mmap_write_lock_killable(me->mm))
> +			return -EINTR;
> +
> +		if (arg2) {
> +			int err = ksm_add_mm(me->mm);
> +
> +			if (!err)
> +				ksm_add_vmas(me->mm);

in the last version of this patch, you reported the error.  Now you
swallow the error.  I have no idea which is correct, but you've
changed the behaviour without explaining it, so I assume it's wrong.

> +		} else {
> +			clear_bit(MMF_VM_MERGE_ANY, &me->mm->flags);
> +		}
> +		mmap_write_unlock(me->mm);
> +		break;
> +	case PR_GET_MEMORY_MERGE:
> +		if (arg2 || arg3 || arg4 || arg5)
> +			return -EINVAL;
> +
> +		error = !!test_bit(MMF_VM_MERGE_ANY, &me->mm->flags);
> +		break;

Why do we need a GET?  Just for symmetry, or is there an actual need for
it?