linux-kernel - Re: [PATCH] mm/hugetlb: correct demote page offset logic

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Message-ID: <efd8008a-6b81-ff4c-f0bd-4f957f00295e@gmail.com>
Date:   Wed, 14 Sep 2022 14:49:20 -0700
From:   Doug Berger <opendmb@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Oscar Salvador <osalvador@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: correct demote page offset logic

On 9/14/2022 1:49 PM, Andrew Morton wrote:
> On Wed, 14 Sep 2022 12:09:17 -0700 Doug Berger <opendmb@...il.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