linux-kernel - Re: [PATCH v4] mm: fix is_pinnable

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Date:   Wed, 11 May 2022 17:12:04 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Dias <joaodias@...gle.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

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