Re: [PATCH 1/4] mm/memory-failure.c: fix race with changing page compound again

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Mon, Feb 28, 2022 at 10:02:42PM +0800, Miaohe Lin wrote:
> There is a race window where we got the compound_head, the hugetlb page
> could be freed to buddy, or even changed to another compound page just
> before we try to get hwpoison page. If this happens, just bail out.

I think that when some hugetlb page is about to change into other type/size
of compound page, it has to go through buddy allocator because hugetlb pages
are maintained in separate memory allocator and they never change into other
normal state directly.  memory_failure_hugetlb() takes refcount before
lock_page(), so the hugetlb page seems not change between get_hwpoison_page()
and lock_page(). So it this new check really necessary?

Thanks,
Naoya Horiguchi

> 
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  mm/memory-failure.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5444a8ef4867..0d7c58340a98 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1534,6 +1534,17 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
>  	}
>  
>  	lock_page(head);
> +
> +	/**
> +	 * The page could have changed compound pages due to race window.
> +	 * If this happens just bail out.
> +	 */
> +	if (!PageHuge(p) || compound_head(p) != head) {
> +		action_result(pfn, MF_MSG_DIFFERENT_COMPOUND, MF_IGNORED);
> +		res = -EBUSY;
> +		goto out;
> +	}
> +
>  	page_flags = head->flags;
>  
>  	if (hwpoison_filter(p)) {
> -- 
> 2.23.0