linux-kernel - Re: [PATCH v2 0/2] mm,fork,security: introduce MADV

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Date:   Tue, 08 Aug 2017 14:45:14 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Florian Weimer <fweimer@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        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 v2 0/2] mm,fork,security: introduce MADV_WIPEONFORK

On Tue, 2017-08-08 at 09:52 -0700, Matthew Wilcox wrote:
> On Tue, Aug 08, 2017 at 11:46:08AM -0400, Rik van Riel wrote:
> > On Tue, 2017-08-08 at 08:19 -0700, Mike Kravetz wrote:
> > > If the use case is fairly specific, then perhaps it makes sense
> > > to
> > > make MADV_WIPEONFORK not applicable (EINVAL) for mappings where
> > > the
> > > result is 'questionable'.
> > 
> > That would be a question for Florian and Colm.
> > 
> > If they are OK with MADV_WIPEONFORK only working on
> > anonymous VMAs (no file mapping), that certainly could
> > be implemented.
> > 
> > On the other hand, I am not sure that introducing cases
> > where MADV_WIPEONFORK does not implement wipe-on-fork
> > semantics would reduce user confusion...
> 
> It'll simply do exactly what it does today, so it won't introduce any
> new fallback code.

Sure, but actually implementing MADV_WIPEONFORK in a
way that turns file mapped VMAs into zero page backed
anonymous VMAs after fork takes no more code than
implementing it in a way that refuses to work on VMAs
that have a file backing.

There is no complexity argument for or against either
approach.

The big question is, what is the best for users?

Should we return -EINVAL when MADV_WIPEONFORK is called
on a VMA that has a file backing, and only succeed on
anonymous VMAs?

Or, should we simply turn every memory range that has
MADV_WIPEONFORK done to it into an anonymous VMA in the
child process?