From: John Hubbard <jhubbard@nvidia.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
John Dias <joaodias@google.com>,
David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page
Date: Tue, 10 May 2022 16:58:13 -0700 [thread overview]
Message-ID: <bb6e76f1-2cbc-893d-c8ab-3ecd3fcae2a5@nvidia.com> (raw)
In-Reply-To: <Ynr10y6pkH6WwbQo@google.com>
On 5/10/22 4:31 PM, Minchan Kim wrote:
>>> + int __mt = get_pageblock_migratetype(page);
>>> + int mt = __READ_ONCE(__mt);
>>
>> Although I saw the email discussion about this in v2, that discussion
>> didn't go far enough. It started with "don't use volatile", and went
>> on to "try __READ_ONCE() instead", but it should have continued on
>> to "you don't need this at all".
>
> That's really what I want to hear from experts so wanted to learn
> "Why". How could we prevent refetching of the mt if we don't use
> __READ_ONCE or volatile there?
>
>>
>> Because you don't. There is nothing you are racing with, and adding
>> __READ_ONCE() in order to avoid a completely not-going-to-happen
>> compiler re-invocation of a significant code block is just very wrong.
>>
>> So let's just let it go entirely. :)
>
> Yeah, once it's clear for everyone, I am happy to remove the
> unnecessary lines.
>
>>
>>> +
>>> + if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
>>
With or without __READ_ONCE() or volatile or anything else,
this code will do what you want. Which is: loosely check
for either of the above.
What functional problem do you think you are preventing
with __READ_ONCE()? Because I don't see one.
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2022-05-10 23:59 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-10 21:17 [PATCH v4] mm: fix is_pinnable_page against on cma page Minchan Kim
2022-05-10 22:56 ` John Hubbard
2022-05-10 23:31 ` Minchan Kim
2022-05-10 23:58 ` John Hubbard [this message]
2022-05-11 0:09 ` Minchan Kim
2022-05-11 4:32 ` John Hubbard
2022-05-11 21:46 ` Minchan Kim
2022-05-11 22:25 ` John Hubbard
2022-05-11 22:37 ` Minchan Kim
2022-05-11 22:49 ` John Hubbard
2022-05-11 23:08 ` Minchan Kim
2022-05-11 23:13 ` John Hubbard
2022-05-11 23:15 ` Minchan Kim
2022-05-11 23:28 ` Minchan Kim
2022-05-11 23:33 ` John Hubbard
2022-05-11 23:45 ` Paul E. McKenney
2022-05-11 23:57 ` John Hubbard
2022-05-12 0:12 ` Paul E. McKenney
2022-05-12 0:12 ` John Hubbard
2022-05-12 0:22 ` Paul E. McKenney
2022-05-12 0:26 ` Minchan Kim
2022-05-12 0:34 ` John Hubbard
2022-05-12 0:49 ` Paul E. McKenney
2022-05-12 1:02 ` John Hubbard
2022-05-12 1:03 ` Minchan Kim
2022-05-12 1:08 ` John Hubbard
2022-05-12 2:18 ` John Hubbard
2022-05-12 3:44 ` Minchan Kim
2022-05-12 4:47 ` John Hubbard
2022-05-17 14:00 ` Jason Gunthorpe
2022-05-17 18:12 ` John Hubbard
2022-05-17 19:28 ` Jason Gunthorpe
2022-05-17 20:12 ` John Hubbard
2022-05-17 20:21 ` Paul E. McKenney
2022-05-23 16:33 ` Minchan Kim
2022-05-24 2:55 ` John Hubbard
2022-05-24 5:16 ` Minchan Kim
2022-05-24 6:22 ` John Hubbard
2022-05-24 14:19 ` Jason Gunthorpe
2022-05-24 15:43 ` Minchan Kim
2022-05-24 15:48 ` Jason Gunthorpe
2022-05-24 16:37 ` Paul E. McKenney
2022-05-24 16:59 ` Minchan Kim
2022-05-12 3:57 ` Paul E. McKenney
2022-05-12 1:03 ` Minchan Kim
2022-05-12 0:35 ` Paul E. McKenney
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