Re: put_page() tries to handle hugepages but fails

[Andrew Morton <akpm@osdl.org>]

David Gibson <david@gibson.dropbear.id.au> wrote:
>
> 1) put_page() can still theoretically call
>  __page_cache_release() which is wrong (and makes the code misleading)

Don't think so?

void put_page(struct page *page)
{
	if (unlikely(PageCompound(page))) {
		page = (struct page *)page->private;
		if (put_page_testzero(page)) {
			void (*dtor)(struct page *page);

			dtor = (void (*)(struct page *))page[1].mapping;
			(*dtor)(page);
		}
		return;
	}
	if (!PageReserved(page) && put_page_testzero(page))
		__page_cache_release(page);
}

>  - patch below replaces this with a BUG() if there is no destructor. 2)
>  what about wli's concern that mapping may be accessed without first
>  checking for a PageCompound?

That shouldn't be happening (should it?).  If someone had such a page
they'd need to lock it to play with ->mapping and I cannot think of any
code which does all that and yet could stumble across a compound page?