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Subject: Re: [PATCH v2 0/2] mm,fork,security: introduce MADV_WIPEONFORK
To: Michal Hocko <mhocko@kernel.org>
Cc: =?UTF-8?Q?Colm_MacC=c3=a1rthaigh?= <colm@allcosts.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>, Rik van Riel <riel@redhat.com>,
        Will Drewry <wad@chromium.org>, akpm@linux-foundation.org,
        dave.hansen@intel.com, kirill@shutemov.name, linux-api@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, luto@amacapital.net,
        mingo@kernel.org
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From: Florian Weimer <fweimer@redhat.com>
Message-ID: <6a04f59b-b72b-c468-ea5c-230764a24402@redhat.com>
Date: Fri, 11 Aug 2017 17:24:29 +0200
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On 08/11/2017 04:24 PM, Michal Hocko wrote:
> On Fri 11-08-17 16:11:44, Florian Weimer wrote:
>> On 08/11/2017 04:06 PM, Michal Hocko wrote:
>>
>>> I am sorry to look too insisting here (I have still hard time to reconcile
>>> myself with the madvise (ab)use) but if we in fact want minherit like
>>> interface why don't we simply add minherit and make the code which wants
>>> to use that interface easier to port? Is the only reason that hooking
>>> into madvise is less code? If yes is that a sufficient reason to justify
>>> the (ab)use of madvise? If there is a general consensus on that part I
>>> will shut up and won't object anymore. Arguably MADV_DONTFORK would fit
>>> into minherit API better as well.
>>
>> It does, OpenBSD calls it MAP_INHERIT_NONE.
>>
>> Could you implement MAP_INHERIT_COPY and MAP_INHERIT_SHARE as well?  Or
>> is changing from MAP_SHARED to MAP_PRIVATE and back impossible?
> 
> I haven't explored those two very much. Their semantic seems rather
> awkward, especially map_inherit_share one. I guess MAP_INHERIT_COPY
> would be doable. Do we have to support all modes or a missing support
> would disqualify the syscall completely?

I think it would be a bit awkward if we implemented MAP_INHERIT_ZERO and
it would not turn a shared mapping into a private mapping in the child,
or would not work on shared mappings at all, or deviate in any way from
the OpenBSD implementation.

MAP_INHERIT_SHARE for a MAP_PRIVATE mapping which has been modified is a
bit bizarre, and I don't know how OpenBSD implements any of this.  It
could well be that the exact behavior implemented in OpenBSD is a poor
fit for the Linux VM implementation.

Florian