linux-kernel - Re: [PATCH 2/3] Add VM_INIT_DEF_MASK and PRCTL_THP

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Date:	Mon, 3 Feb 2014 18:46:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Alex Thorlton <athorlton@....com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Jiang Liu <liuj97@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Robin Holt <holt@....com>, Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	liguang <lig.fnst@...fujitsu.com>, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] Add VM_INIT_DEF_MASK and PRCTL_THP_DISABLE

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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