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

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Message-ID: <1219032d-5ee2-bec5-3975-ae1abcc3bfa0@nvidia.com>
Date:   Wed, 11 May 2022 16:33:38 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.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 16:28, 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@...dia.com>


thanks,
-- 
John Hubbard
NVIDIA