Re: [PATCH v4 2/3] mm/memory-failure: add panic option for unrecoverable pages

[Miaohe Lin]

On 2026/4/24 20:01, Breno Leitao wrote:
> On Thu, Apr 23, 2026 at 10:38:19AM +0800, Miaohe Lin wrote:
>>> are you suggesting I drop MF_MSG_KERNEL_HIGH_ORDER from here, or, document this
>>> will not hit userspace pages?
>>
>> No, maybe we should rule out or document above rare case if I'm not miss something.
> 
> Good catch. A buddy page being concurrently allocated to userspace can
> briefly satisfy get_hwpoison_page() == 0 && !is_free_buddy_page(), and
> that page is recoverable via the standard SIGBUS path — panicking on
> it would be wrong.
> 
> The page allocator can't filter it out either.
> 
> check_new_pages() is gated by is_check_pages_enabled() and is a no-op
> when CONFIG_DEBUG_VM=n.
> 
> For v6 I'll try to rule out the race inside panic_on_unrecoverable_mf() so
> action_result() stays unchanged:
> 
> 	case MF_MSG_KERNEL_HIGH_ORDER:
> 	p = pfn_to_online_page(pfn);
> 	if (!p)
> 		return true;
> 	cpu_relax();
> 	return page_count(p) == 0 &&
> 		!PageLRU(p) &&
> 		!page_mapped(p) &&
> 		!page_folio(p)->mapping &&
> 		!is_free_buddy_page(p);
> 
> 
> A buddy page being allocated must transit rmqueue() → prep_new_page() →
> post_alloc_hook() before the caller can use it. Each step either bumps
> _refcount or sets state we can observe (PageLRU, ->mapping). cpu_relax()
> lets that remote-CPU progress become visible before we resample.
> 
> A genuine non-buddy high-order kernel tail page stays unowned across the
> recheck, so the panic still fires on the case this series targets.
> 
> The window is much narrowed now, not eliminated — I'll say so in the changelog.
> 
> I also added a selftest that enables the sysctl, injects MADV_HWPOISON
> on a userspace anon page in a forked child, and asserts SIGBUS (not a
> panic). I've been running this in a loop for hours, and I haven't seen any
> false positive.

The userspace anon pages are already allocated. Those pages are in a stable state.
So your selftest cannot test above window. Or am I miss something?

Thanks.
.