linux-kernel - Re: [PATCH v3 1/7] kernel/fork: convert vma assignment to a memcpy

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Message-ID: <20230126172726.GA682281@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 26 Jan 2023 09:27:26 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...hsingularity.net, dave@...olabs.net,
        willy@...radead.org, liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, mingo@...hat.com, will@...nel.org,
        luto@...nel.org, songliubraving@...com, peterx@...hat.com,
        david@...hat.com, dhowells@...hat.com, hughd@...gle.com,
        bigeasy@...utronix.de, kent.overstreet@...ux.dev,
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        linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH v3 1/7] kernel/fork: convert vma assignment to a memcpy

On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote:
> On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@...gle.com> wrote:
> 
> > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > >
> > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@...gle.com> wrote:
> > >
> > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler
> > > > errors when we add a const modifier to vma->vm_flags.
> > > >
> > > > ...
> > > >
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> > > >                * orig->shared.rb may be modified concurrently, but the clone
> > > >                * will be reinitialized.
> > > >                */
> > > > -             *new = data_race(*orig);
> > > > +             memcpy(new, orig, sizeof(*new));
> > >
> > > The data_race() removal is unchangelogged?
> > 
> > True. I'll add a note in the changelog about that. Ideally I would
> > like to preserve it but I could not find a way to do that.
> 
> Perhaps Paul can comment?
> 
> I wonder if KCSAN knows how to detect this race, given that it's now in
> a memcpy.  I assume so.

I ran an experiment memcpy()ing between a static array and an onstack
array, and KCSAN did not complain.  But maybe I was setting it up wrong.

This is what I did:

	long myid = (long)arg; /* different value for each task */
	static unsigned long z1[10] = { 0 };
	unsigned long z2[10];

	...

	memcpy(z1, z2, ARRAY_SIZE(z1) * sizeof(z1[0]));
	for (zi = 0; zi < ARRAY_SIZE(z1); zi++)
		z2[zi] += myid;
	memcpy(z2, z1, ARRAY_SIZE(z1) * sizeof(z1[0]));

Adding Marco on CC for his thoughts.

						Thanx, Paul