Re: [PATCH] hwpoison: Fix race with changing page during offlining v2

Andrew Morton
2014-07-01T22:27:19Z

On Mon, 30 Jun 2014 17:32:16 -0700 Andi Kleen <andi@firstfloor.org> wrote:

> From: Andi Kleen <ak@linux.intel.com>
>
> When a hwpoison page is locked it could change state
> due to parallel modifications. Check after the lock
> if the page is still the same compound page.
>
> ...
>
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1168,6 +1168,16 @@ int memory_failure(unsigned long pfn, int trapno, int flags)
> lock_page(hpage);
>
> /*
> + * The page could have changed compound pages during the locking.
> + * If this happens just bail out.
> + */
> + if (compound_head(p) != hpage) {

How can a 4k page change compound pages? The original compound page
was torn down and then this 4k page became part of a differently-sized
compound page?

> + action_result(pfn, "different compound page after locking", IGNORED);
> + res = -EBUSY;
> + goto out;
> + }
> +
> + /*

I don't get it. We just go and fail the poisoning attempt? Shouldn't
we go back, grab the new hpage and try again?