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

John Hubbard
2022-05-11T23:56:43Z

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