linux-kernel - Re: [PATCH 2/2] mm,fork: introduce MADV

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Date:   Fri, 11 Aug 2017 10:07:04 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Rik van Riel <riel@...hat.com>, Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        fweimer@...hat.com, colm@...costs.net, akpm@...ux-foundation.org,
        keescook@...omium.org, luto@...capital.net, wad@...omium.org,
        mingo@...nel.org, kirill@...temov.name, dave.hansen@...el.com
Subject: Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

On 08/11/2017 09:59 AM, Rik van Riel wrote:
> On Fri, 2017-08-11 at 09:36 -0700, Mike Kravetz wrote:
>> 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.
> 
> It worked here, but now I don't understand why :)
> 
>>>
>>> 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.
> 
> In other words (flags & MAP_SHARED || vma->vm_file) would catch
> hugetlbfs, too?

Yes, that should catch hugetlbfs.

-- 
Mike Kravetz