As mentioned in commit c07aea3ef4d4 ("mm: add a signature in struct page"): "The page->signature field is aliased to page->lru.next and page->compound_head."
And as the comment in page_is_pfmemalloc(): "lru.next 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 lru.next is set for a pfmemalloc page. What happens here is that the original pp->signature is OR'ed with PP_SIGNATURE after the allocation in order to preserve any existing bits(such as the bit 1, used to indicate a pfmemalloc page), so when those bits are present, those page is not considered to be from page pool and the DMA mapping of those pages will be left stale.
As bit 0 is for page->compound_head, So mask both bit 0/1 before the checking in page_pool_return_skb_page(). And we will return those pfmemalloc pages back to the page allocator after cleaning up the DMA mapping.
Fixes: 6a5bcd84e886 ("page_pool: Allow drivers to hint on SKB recycling") Reviewed-by: Ilias Apalodimas ilias.apalodimas@linaro.org Signed-off-by: Yunsheng Lin linyunsheng@huawei.com --- V2: explain more on why we need to mask those bits. --- net/core/page_pool.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/net/core/page_pool.c b/net/core/page_pool.c index 5e4eb45..8ab7b40 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c @@ -634,7 +634,15 @@ bool page_pool_return_skb_page(struct page *page) struct page_pool *pp;
page = compound_head(page); - if (unlikely(page->pp_magic != PP_SIGNATURE)) + + /* page->pp_magic is OR'ed with PP_SIGNATURE after the allocation + * in order to preserve any existing bits, such as bit 0 for the + * head page of compound page and bit 1 for pfmemalloc page, so + * mask those bits for freeing side when doing below checking, + * and page_is_pfmemalloc() is checked in __page_pool_put_page() + * to avoid recycling the pfmemalloc page. + */ + if (unlikely((page->pp_magic & ~0x3UL) != PP_SIGNATURE)) return false;
pp = page->pp;