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

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Date:   Tue, 15 Aug 2017 22:18:19 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, mhocko@...nel.org,
        mike.kravetz@...cle.com, linux-mm@...ck.org, fweimer@...hat.com,
        colm@...costs.net, keescook@...omium.org, luto@...capital.net,
        wad@...omium.org, mingo@...nel.org, kirill@...temov.name,
        dave.hansen@...el.com, linux-api@...r.kernel.org,
        torvalds@...ux-foundation.org, willy@...radead.org
Subject: Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

On Tue, 2017-08-15 at 15:51 -0700, Andrew Morton wrote:
> On Fri, 11 Aug 2017 17:28:29 -0400 riel@...hat.com wrote:
> 
> > A further complication is the proliferation of clone flags,
> > programs bypassing glibc's functions to call clone directly,
> > and programs calling unshare, causing the glibc pthread_atfork
> > hook to not get called.
> > 
> > It would be better to have the kernel take care of this
> > automatically.
> 
> I'll add "The patch also adds MADV_KEEPONFORK, to undo the effects of
> a
> prior MADV_WIPEONFORK." here.
> 
> I guess it isn't worth mentioning that these things can cause VMA
> merges and splits. 

That's the same as every other Linux specific madvise operation.

> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -80,6 +80,17 @@ static long madvise_behavior(struct
> > vm_area_struct *vma,
> >  		}
> >  		new_flags &= ~VM_DONTCOPY;
> >  		break;
> > +	case MADV_WIPEONFORK:
> > +		/* MADV_WIPEONFORK is only supported on anonymous
> > memory. */
> > +		if (vma->vm_file || vma->vm_flags & VM_SHARED) {
> > +			error = -EINVAL;
> > +			goto out;
> > +		}
> > +		new_flags |= VM_WIPEONFORK;
> > +		break;
> > +	case MADV_KEEPONFORK:
> > +		new_flags &= ~VM_WIPEONFORK;
> > +		break;
> >  	case MADV_DONTDUMP:
> >  		new_flags |= VM_DONTDUMP;
> >  		break;
> 
> It seems odd to permit MADV_KEEPONFORK against other-than-anon vmas?

Given that the only way to set VM_WIPEONFORK is through
MADV_WIPEONFORK, calling MADV_KEEPONFORK on an
other-than-anon vma would be equivalent to a noop.

If new_flags == vma->vm_flags, madvise_behavior() will
immediately exit.