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

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Date:   Fri, 11 Aug 2017 11:23:12 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, mike.kravetz@...cle.com,
        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 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.

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.