Re: [PATCH net] page_pool: mask the page->signature before the checking

From: Matthew Wilcox
Date: Wed Aug 04 2021 - 21:51:18 EST

On Thu, Aug 05, 2021 at 09:06:57AM +0800, Yunsheng Lin wrote:
> As mentioned in commit c07aea3ef4d4 ("mm: add a signature
> in struct page"):
> "The page->signature field is aliased to page-> and
> page->compound_head."
> And as the comment in page_is_pfmemalloc():
> " has bit 1 set if the page is allocated from the
> pfmemalloc reserves. Callers may simply overwrite it if they
> do not need to preserve that information."
> The page->signature is or’ed with PP_SIGNATURE when a page is
> allocated in page pool, see __page_pool_alloc_pages_slow(),
> and page->signature is checked directly with PP_SIGNATURE in
> page_pool_return_skb_page(), which might cause resoure leaking
> problem for a page from page pool if bit 1 of is set for
> a pfmemalloc page.
> As bit 0 is page->compound_head, So mask both bit 0 and 1 before
> the checking in page_pool_return_skb_page().

No, you don't understand. We *want* the check to fail if we were low
on memory so we return the emergency allocation.