linux-kernel - Re: [PATCH 1/2] hugetlb: fix update_and_free

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Date:   Wed, 17 Feb 2021 11:02:52 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Zi Yan <ziy@...dia.com>, Davidlohr Bueso <dbueso@...e.de>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oscar Salvador <osalvador@...e.de>,
        Joao Martins <joao.m.martins@...cle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/2] hugetlb: fix update_and_free_page contig page
 struct assumption

On Wed, 17 Feb 2021 10:49:25 -0800 Mike Kravetz <mike.kravetz@...cle.com> wrote:

> page structs are not guaranteed to be contiguous for gigantic pages.  The
> routine update_and_free_page can encounter a gigantic page, yet it assumes
> page structs are contiguous when setting page flags in subpages.
> 
> If update_and_free_page encounters non-contiguous page structs, we can
> see “BUG: Bad page state in process …” errors.
> 
> Non-contiguous page structs are generally not an issue.  However, they can
> exist with a specific kernel configuration and hotplug operations.  For
> example: Configure the kernel with CONFIG_SPARSEMEM and
> !CONFIG_SPARSEMEM_VMEMMAP.  Then, hotplug add memory for the area where the
> gigantic page will be allocated.
> Zi Yan outlined steps to reproduce here [1].
> 
> [1] https://lore.kernel.org/linux-mm/16F7C58B-4D79-41C5-9B64-A1A1628F4AF2@nvidia.com/
> 
> Fixes: 944d9fec8d7a ("hugetlb: add support for gigantic page allocation at runtime")

June 2014.  That's a long lurk time for a bug.  I wonder if some later
commit revealed it.

I guess it doesn't matter a lot, but some -stable kernel maintainers
might wonder if they really need this fix...