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

Paul E. McKenney
2022-05-12T00:12:17Z

On Wed, May 11, 2022 at 04:57:04PM -0700, 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? :)

Apparently there is not much sacred to compiler writers in search of
additional optimizations. :-/

> > 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?

Not yet. But if link-time optimizations (LTO) continue their march,
I wouldn't feel safe ruling it out...

> 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.

...especially if the code is reorganized for whatever reason.

> thanks,
> --
> John Hubbard
> NVIDIA

But again:

> > In the end, it is your code, so you get to decide how much you would
> > like to keep track of what compilers get up to over time. ;-)

Thanx, Paul