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

John Hubbard
2022-05-11T23:33:16Z

On 5/11/22 16:28, Minchan Kim wrote:
> This is delta to confirm before posting next revision.
>
> Are you okay with this one?

Yes, just maybe delete the comment:

>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index cbf79eb790e0..7b2df6780552 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1626,14 +1626,14 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
> static inline bool is_pinnable_page(struct page *page)
> {
> #ifdef CONFIG_CMA
> + int mt = get_pageblock_migratetype(page);
> +
> /*
> - * use volatile to use local variable mt instead of
> - * refetching mt value.
> + * "&" operation would prevent compiler split up
> + * get_pageblock_migratetype two times for each
> + * condition check: refetching mt value two times.
> */

If you wanted a useful comment here, I think a description of
what is running at the same time would be good. But otherwise,
I'd just delete the entire comment, because a micro-comment
about "&" is not really worth the vertical space here IMHO.

> - int __mt = get_pageblock_migratetype(page);
> - int mt = __READ_ONCE(__mt);
> -
> - if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
> + if (mt & (MIGRATE_ISOLATE | MIGRATE_CMA))
> return false;
> #endif
>

Anyway, I'm comfortable with this now:

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
--
John Hubbard
NVIDIA