linux-kernel - Re: [PATCH v2 4/8] mm/gup: track FOLL

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Message-ID: <1037b514-30e7-c128-38c1-9c98488be337@nvidia.com>
Date:   Thu, 30 Jan 2020 19:19:30 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-rdma@...r.kernel.org>, <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v2 4/8] mm/gup: track FOLL_PIN pages

On 1/30/20 3:31 AM, Kirill A. Shutemov wrote:
...
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 0a55dec68925..b1079aaa6f24 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>>>>   	 */
>>>>   	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
>>>> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
>>>> +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
>>>> +			 (FOLL_PIN | FOLL_GET)))
>>>
>>> Too many parentheses.
>>
>>
>> OK, I'll remove at least one. :)
> 
> I see two.

ah, correction: actually, the original statement has exactly the right number of parentheses.
The relevant C precedence order is:
	==
	&
	|

...which means that both "&" and "|" operations need parentheses protection from the higher
precedence "==" operation.

(There are other places in the kernel that have this exact pattern, too, with the same
pattern of parentheses that I'm using, of course.)

thanks,
-- 
John Hubbard
NVIDIA