Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

[Mike Kravetz]

On 08/11/2017 08:23 AM, Rik van Riel wrote:
> On Thu, 2017-08-10 at 17:23 +0200, Michal Hocko wrote:
>> On Sun 06-08-17 10:04:25, Rik van Riel wrote:
>> [...]
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index 17921b0390b4..db1fb2802ecc 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -659,6 +659,13 @@ static __latent_entropy int dup_mmap(struct
>>> mm_struct *mm,
>>>  		tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
>>>  		tmp->vm_next = tmp->vm_prev = NULL;
>>>  		file = tmp->vm_file;
>>> +
>>> +		/* With VM_WIPEONFORK, the child gets an empty
>>> VMA. */
>>> +		if (tmp->vm_flags & VM_WIPEONFORK) {
>>> +			tmp->vm_file = file = NULL;
>>> +			tmp->vm_ops = NULL;
>>> +		}
>>
>> What about VM_SHARED/|VM)MAYSHARE flags. Is it OK to keep the around?
>> At
>> least do_anonymous_page SIGBUS on !vm_ops && VM_SHARED. Or do I miss
>> where those flags are cleared?
> 
> Huh, good spotting.  That makes me wonder why the test case that
> Mike and I ran worked just fine on a MAP_SHARED|MAP_ANONYMOUS VMA,
> and returned zero-filled memory when read by the child process.

Well, I think I still got a BUG with a MAP_SHARED|MAP_ANONYMOUS vma on
your v2 patch.  Did not really want to start a discussion on the
implementation until the issue of exactly what VM_WIPEONFORK was supposed
to do was settled.

> 
> OK, I'll do a minimal implementation for now, which will return
> -EINVAL if MADV_WIPEONFORK is called on a VMA with MAP_SHARED
> and/or an mmapped file.
> 
> It will work the way it is supposed to with anonymous MAP_PRIVATE
> memory, which is likely the only memory it will be used on, anyway.
> 

Seems reasonable.

You should also add VM_HUGETLB to those returning -EINVAL.  IIRC, a
VM_HUGETLB vma even without VM_SHARED expects vm_file != NULL.

-- 
Mike Kravetz