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

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Message-ID: <0d90390c-3624-4f93-f8bd-fb29e92237d3@nvidia.com>
Date:   Wed, 11 May 2022 17:12:32 -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 16:57, 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