linux-kernel - Re: linux-next: build warning after merge of the akpm-current tree

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Message-ID: <ed7a058f-ca84-2f0f-4d22-ce1eb8ce0645@oracle.com>
Date:   Wed, 1 Apr 2020 16:06:45 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Jason Gunthorpe <jgg@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Longpeng <longpeng2@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: linux-next: build warning after merge of the akpm-current tree

On 4/1/20 4:00 PM, Jason Gunthorpe wrote:
> On Wed, Apr 01, 2020 at 03:58:31PM -0700, Andrew Morton wrote:
>> On Tue, 31 Mar 2020 19:56:12 +1100 Stephen Rothwell <sfr@...b.auug.org.au> wrote:
>>
>>> Hi all,
>>>
>>> After merging the akpm-current tree, today's linux-next build (i386
>>> defconfig) produced this warning:
>>>
>>> mm/hugetlb.c: In function 'huge_pte_offset':
>>> cc1: warning: function may return address of local variable [-Wreturn-local-addr]
>>> mm/hugetlb.c:5361:14: note: declared here
>>>  5361 |  pud_t *pud, pud_entry;
>>>       |              ^~~~~~~~~
>>> cc1: warning: function may return address of local variable [-Wreturn-local-addr]
>>> mm/hugetlb.c:5360:14: note: declared here
>>>  5360 |  p4d_t *p4d, p4d_entry;
>>>       |              ^~~~~~~~~
>>>
>>> Introduced by commit
>>>
>>>   826ddc88e2cf ("mm/hugetlb: fix a addressing exception caused by huge_pte_offset")
>>
>> I can reproduce this (i386 defconfig, gcc-7.2.0).
>>
>> I can see no way in which this makes any sense.  Hopefully it's a gcc
>> bug but it's hard to see how it could have messed up this fairly simple
>> code.
> 
> It is a code bug, there was a little thread about this. It happens
> because the address of a stack variable is passed into the pXX_offset
> functions which return that address when the page levels are folded.
> 
> I recommend to drop this patch until it is adjusted..

Yes, this patch causes at least the BUG here,

https://lore.kernel.org/linux-mm/CA+G9fYsJgZhhWLMzUxu_ZQ+THdCcJmFbHQ2ETA_YPP8M6yxOYA@mail.gmail.com/

-- 
Mike Kravetz