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

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Message-ID: <8f083802-7ab0-15ec-b37d-bc9471eea0b1@nvidia.com>
Date:   Wed, 11 May 2022 16:13:10 -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 16:08, Minchan Kim wrote:
>> OK, so the code checks the wrong item each time. But the code really
>> only needs to know "is either _CMA or _ISOLATE set?". And so you
> 
> Yes.
> 
>> can just sidestep the entire question by writing it like this:
>>
>> int mt = get_pageblock_migratetype(page);
>>
>> if (mt & (MIGRATE_ISOLATE | MIGRATE_CMA))
>> 	return false;
> 
> I am confused. Isn't it same question?
> 
>                                                      set_pageblock_migratetype(MIGRATE_ISOLATE)
> if (get_pageblock_migrate(page) & MIGRATE_CMA)
> 
>                                                      set_pageblock_migratetype(MIGRATE_CMA)
> 
> if (get_pageblock_migrate(page) & MIGRATE_ISOLATE)

Well no, because the "&" operation is a single operation on the CPU, and 
isn't going to get split up like that.


thanks,
-- 
John Hubbard
NVIDIA