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Message-ID: <efd8008a-6b81-ff4c-f0bd-4f957f00295e@gmail.com>
Date:   Wed, 14 Sep 2022 14:49:20 -0700
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Subject: Re: [PATCH] mm/hugetlb: correct demote page offset logic
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To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Oscar Salvador <osalvador@suse.de>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
References: <20220914190917.3517663-1-opendmb@gmail.com>
 <20220914134927.16c229ccdc1a6b9da5d698c3@linux-foundation.org>
From:   Doug Berger <opendmb@gmail.com>
In-Reply-To: <20220914134927.16c229ccdc1a6b9da5d698c3@linux-foundation.org>
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On 9/14/2022 1:49 PM, Andrew Morton wrote:
> On Wed, 14 Sep 2022 12:09:17 -0700 Doug Berger <opendmb@gmail.com> wrote:
> 
>> With gigantic pages it may not be true that struct page structures
>> are contiguous across the entire gigantic page. The nth_page macro
>> is used here in place of direct pointer arithmetic to correct for
>> this.
> 
> What were the user-visible runtime effects of this bug?
As Mike said this would only conceptually be a problem for systems with 
CONFIG_SPARSEMEM && !CONFIG_SPARSEMEM_VMEMMAP, and could cause kernel 
address exceptions or memory corruption with unpredictable side effects.

However, I am unaware of a system other than perhaps the PS3 that uses 
the classic sparse addressing, so the odds of such a system also using 
gigantic hugetlbfs pages that it wants to demote is likely quite small.

Thanks,
-Doug