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Message-ID: <1502470781.6577.49.camel@redhat.com>
Subject: Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK
From: Rik van Riel <riel@redhat.com>
To: Mike Kravetz <mike.kravetz@oracle.com>,
        Michal Hocko <mhocko@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, fweimer@redhat.com,
        colm@allcosts.net, akpm@linux-foundation.org, keescook@chromium.org,
        luto@amacapital.net, wad@chromium.org, mingo@kernel.org,
        kirill@shutemov.name, dave.hansen@intel.com
Date: Fri, 11 Aug 2017 12:59:41 -0400
In-Reply-To: <6a3e2dbe-6274-4402-0716-88f4fbda73dd@oracle.com>
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         <20170810152352.GZ23863@dhcp22.suse.cz>
         <1502464992.6577.48.camel@redhat.com>
         <6a3e2dbe-6274-4402-0716-88f4fbda73dd@oracle.com>
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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?