Re: [PATCH RFC 18/19] mm/page_alloc: implement __GFP_UNMAPPED|__GFP_ZERO allocations

[Brendan Jackman <jackmanb@google.com>]

On Wed Feb 25, 2026 at 4:34 PM UTC, Brendan Jackman wrote:

> +static inline void clear_page_mermap(struct page *page, unsigned int numpages)
> +{
> +	void *mermap;
> +
> +	BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHMEM));
> +
> +	/* Fast path: single mapping (may fail under preemption). */
> +	mermap = mermap_get(page, numpages << PAGE_SHIFT, PAGE_KERNEL_NOGLOBAL);
> +	if (mermap) {
> +		void *buf = kasan_reset_tag(mermap_addr(mermap));
> +
> +		for (int i = 0; i < numpages; i++)
> +			clear_page(buf + (i << PAGE_SHIFT));
> +		mermap_put(mermap);
> +		return;
> +	}
> +
> +	/* Slow path, map each page individually (always succeeds). */
> +	for (int i = 0; i < numpages; i++) {
> +		unsigned long flags;
> +
> +		local_irq_save(flags);
> +		mermap = mermap_get_reserved(page + i, PAGE_KERNEL);

Oh, that should be PAGE_KERNEL_NONGLOBAL. I think this means I never
tested this path as this will crash - mermap_get_reserved() will WARN()
and return NULL and then we'll dereference that below. 

Maybe that's a bad idea, maybe it should WARN() but serve the request
anyway.

Anyway maybe I need to support failure injection to test the fallback path.

> +		clear_page(kasan_reset_tag(mermap_addr(mermap)));
> +		mermap_put(mermap);
> +		local_irq_restore(flags);
> +	}
> +}
> +#else
> +static inline bool pageblock_unmapped(struct page *page)
> +{
> +	return false;
> +}
> +
> +static inline void clear_page_mermap(struct page *page, unsigned int numpages)
> +{
> +	BUG();
> +}
> +#endif
> +
> +static void kernel_init_pages(struct page *page, unsigned int numpages)
> +{
> +	int num_blocks = DIV_ROUND_UP(numpages, pageblock_nr_pages);
> +
> +	for (int block = 0; block < num_blocks; block++) {
> +		struct page *block_page = page + (block << pageblock_order);
> +		bool unmapped = pageblock_unmapped(block_page);
> +
> +		/* s390's use of memset() could override KASAN redzones. */
> +		kasan_disable_current();
> +		if (unmapped) {
> +			clear_page_mermap(page, numpages);

That should be clearing block_page.

> +		} else {
> +			for (int i = 0; i < min(numpages, pageblock_nr_pages); i++)
> +				clear_highpage_kasan_tagged(block_page + i);
> +		}
> +		kasan_enable_current();
> +
> +		numpages -= pageblock_nr_pages;
> +	}
>  }