On 2/17/21 11:02 AM, Andrew Morton wrote:
On Wed, 17 Feb 2021 10:49:25 -0800 Mike Kravetz mike.kravetz@oracle.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...
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...
I am not sure how common a CONFIG_SPARSEMEM and !CONFIG_SPARSEMEM_VMEMMAP config is. On the more popular architectures, this is not the default. But, you can build a kernel with such options. And, then you need to hotplug memory add and allocate a gigantic page there.
It is unlikely to happen, but possible since Zi could force the BUG.
The copy_huge_page_from_user bug requires the same non-normal configuration and is just as unlikely to occurr. But, since it can overwrite somewhat random pages I would feel better if it was fixed.