Re: [PATCH v5 2/4] mm/memory-failure: add panic option for unrecoverable pages

["David Hildenbrand (Arm)" <david@kernel.org>]

> +	switch (type) {
> +	case MF_MSG_KERNEL:
> +	case MF_MSG_UNKNOWN:
> +		return true;
> +	case MF_MSG_KERNEL_HIGH_ORDER:
> +		/*
> +		 * Rule out a concurrent buddy allocation: give the
> +		 * allocator a moment to finish prep_new_page() and
> +		 * re-check. A genuine high-order kernel tail page stays
> +		 * unowned; an in-flight allocation will have bumped the
> +		 * refcount, attached a mapping, or placed the page on
> +		 * an LRU by now.
> +		 */
> +		p = pfn_to_online_page(pfn);
> +		if (!p)
> +			return true;
> +		/*
> +		 * Yield so a concurrent allocator on another CPU can
> +		 * finish prep_new_page() and have its writes become
> +		 * visible before we resample the page state.
> +		 */
> +		cpu_relax();
> +		return page_count(p) == 0 &&
> +		       !PageLRU(p) &&
> +		       !page_mapped(p) &&
> +		       !page_folio(p)->mapping &&
> +		       !is_free_buddy_page(p);

I don't get what you are doing here. The right way to check for a tail page is
not by checking the refcount.

Further, you are not holding a folio reference? If so, calling
page_mapped/folio_mapped is shaky. On concurrent folio split you can trigger a
VM_WARN_ON_FOLIO().


Maybe folio_snapshot() is what you are looking for, if you are in fact not
holding a reference?

-- 
Cheers,

David