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Subject: Re: [mm/gup] 9857a17f20: kernel_BUG_at_include/linux/pagemap.h
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        kernel test robot <oliver.sang@intel.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Matthew Wilcox <willy@infradead.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        kernel test robot <lkp@intel.com>
References: <20210907153736.GF17617@xsang-OptiPlex-9020>
 <CAHk-=wh=4TJ3yN0+qpzDPJcH3KFkVsHcHO1FJjKHjaPTx8UCdQ@mail.gmail.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <513d8734-7156-f3b6-b149-4d2cd83b5742@nvidia.com>
Date:   Tue, 7 Sep 2021 12:10:51 -0700
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On 9/7/21 11:14 AM, Linus Torvalds wrote:
> On Tue, Sep 7, 2021 at 8:20 AM kernel test robot <oliver.sang@intel.com> wrote:
>>
>> FYI, we noticed the following commit (built with clang-14):
>>
>> commit: 9857a17f206f ("mm/gup: remove try_get_page(), call try_get_compound_head() directly")
>> caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
>>
>> [  143.908513][ T3260] kernel BUG at include/linux/pagemap.h:223!
> 
> Ahh, well, yes.
> 
> That commit is clearly buggy, in that the try_get_compound_head() code
> really doesn't work at all for us.
> 
> __page_cache_add_speculative() is not at all the same as
> try_get_page(), and I should have caught on to this as I applied it. I
> just read the explanation, and it sounded believable, but it was
> entirely wrong.
> 
> try_get_page() is literally about that "page ref overflow" case, but
> try_get_compound_head() uses page_cache_add_speculative() which has
> different logic and has those extra "this only works in RCU context"
> logic.
> 
> So that commit was completely bogus, and the "lack of maintenance" was
> not lack of maintenance at all, it was all about entirely different
> semantics.
> 
> Reverted.
> 
>                   Linus

Apologies for the bug! There is a lesson in here, somewhere...


thanks,
-- 
John Hubbard
NVIDIA