linux-kernel - Re: [PATCH] sparc64: Fix gup_huge

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Date:   Fri, 23 Jun 2017 00:45:15 +1000
From:   Julian Calaby <julian.calaby@...il.com>
To:     Nitin Gupta <nitin.m.gupta@...cle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Lorenzo Stoakes <lstoakes@...il.com>, Jan Kara <jack@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        sparclinux <sparclinux@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sparc64: Fix gup_huge_pmd

Hi Nitin,

On Fri, Jun 23, 2017 at 12:37 AM, Nitin Gupta <nitin.m.gupta@...cle.com> wrote:
> Hi Julian,
>
>
> On 6/22/17 3:53 AM, Julian Calaby wrote:
>>
>> On Thu, Jun 22, 2017 at 7:50 AM, Nitin Gupta <nitin.m.gupta@...cle.com>
>> wrote:
>>>
>>> The function assumes that each PMD points to head of a
>>> huge page. This is not correct as a PMD can point to
>>> start of any 8M region with a, say 256M, hugepage. The
>>> fix ensures that it points to the correct head of any PMD
>>> huge page.
>>>
>>> Signed-off-by: Nitin Gupta <nitin.m.gupta@...cle.com>
>>> ---
>>>   arch/sparc/mm/gup.c | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/arch/sparc/mm/gup.c b/arch/sparc/mm/gup.c
>>> index cd0e32b..9116a6f 100644
>>> --- a/arch/sparc/mm/gup.c
>>> +++ b/arch/sparc/mm/gup.c
>>> @@ -80,6 +80,8 @@ static int gup_huge_pmd(pmd_t *pmdp, pmd_t pmd,
>>> unsigned long addr,
>>>          refs = 0;
>>>          head = pmd_page(pmd);
>>>          page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
>>> +       if (PageTail(head))
>>> +               head = compound_head(head);
>>
>> Stupid question: shouldn't this go before the page calculation?
>
>
> No, it should be after page calculation: First, 'head' points to base of
> the PMD page, then 'page' points to an offset within that page. Finally,
> we make sure that head variable points to head of the compound page
> which contains the addr.

Thanks for the explanation, that makes a bit more sense to me.

> I think confusion comes from the use of 'head' for pointing to a
> non-head page. So, maybe it would be more clear to write that part
> of the function this way:
>
>         page = pmd_page(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
>         head = compound_head(page);

More verbose variable names would help too. =)

Thanks,

-- 
Julian Calaby

Email: julian.calaby@...il.com
Profile: http://www.google.com/profiles/julian.calaby/