Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

[Minchan Kim <minchan@kernel.org>]

On Wed, May 11, 2022 at 05:22:07PM -0700, Paul E. McKenney wrote:
> On Wed, May 11, 2022 at 05:12:32PM -0700, John Hubbard wrote:
> > On 5/11/22 16:57, John Hubbard wrote:
> > > On 5/11/22 16:45, Paul E. McKenney wrote:
> > > > > 
> > > > > Well no, because the "&" operation is a single operation on the CPU, and
> > > > > isn't going to get split up like that.
> > > > 
> > > > Chiming in a bit late...
> > > 
> > > Much appreciated!
> > > 
> > > > 
> > > > The usual way that this sort of thing causes trouble is if there is a
> > > > single store instruction that changes the value from MIGRATE_ISOLATE
> > > > to MIGRATE_CMA, and if the compiler decides to fetch twice, AND twice,
> > > 
> > > Doing an AND twice for "x & constant" this definitely blows my mind. Is
> > > nothing sacred? :)
> > > 
> > > > and then combine the results.  This could give a zero outcome where the
> > > > underlying variable never had the value zero.
> > > > 
> > > > Is this sort of thing low probability?
> > > > 
> > > > Definitely.
> > > > 
> > > > Isn't this sort of thing prohibited?
> > > > 
> > > > Definitely not.
> > > > 
> > > > So what you have will likely work for at least a while longer, but it
> > > > is not guaranteed and it forces you to think a lot harder about what
> > > > the current implementations of the compiler can and cannot do to you.
> > > > 
> > > > The following LWN article goes through some of the possible optimizations
> > > > (vandalisms?) in this area: https://lwn.net/Articles/793253/
> > > > 
> > > 
> > > hmm, I don't think we hit any of those  cases, do we? Because here, the
> > > "write" side is via a non-inline function that I just don't believe the
> > > compiler is allowed to call twice. Or is it?
> > > 
> > > Minchan's earlier summary:
> > > 
> > > CPU 0                         CPU1
> > > 
> > > 
> > >                                set_pageblock_migratetype(MIGRATE_ISOLATE)
> > > 
> > > if (get_pageblock_migrate(page) & MIGRATE_CMA)
> > > 
> > >                                set_pageblock_migratetype(MIGRATE_CMA)
> > > 
> > > if (get_pageblock_migrate(page) & MIGRATE_ISOLATE)
> > > 
> > > ...where set_pageblock_migratetype() is not inline.
> > > 
> > > thanks,
> > 
> > Let me try to say this more clearly: I don't think that the following
> > __READ_ONCE() statement can actually help anything, given that
> > get_pageblock_migratetype() is non-inlined:
> > 
> > +	int __mt = get_pageblock_migratetype(page);
> > +	int mt = __READ_ONCE(__mt);
> > +
> > +	if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
> > +		return false;
> > 
> > 
> > Am I missing anything here?
> 
> In the absence of future aggression from link-time optimizations (LTO),
> you are missing nothing.

A thing I want to note is Android kernel uses LTO full mode.