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

[John Hubbard]

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?


thanks,
--
John Hubbard
NVIDIA