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

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Date:   Wed, 11 May 2022 16:57:04 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     <paulmck@...nel.org>
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 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,
-- 
John Hubbard
NVIDIA

> 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