Re: [PATCH 1/2] hugetlb: fix update_and_free_page contig page struct assumption

[Andrew Morton]

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