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

John Hubbard
2022-05-12T00:12:26Z

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