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

[Ilias Apalodimas]

On Thu, Aug 05, 2021 at 10:14:39AM +0800, Yunsheng Lin wrote:
> On 2021/8/5 9:50, Matthew Wilcox wrote:
> > 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->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.
> >>
> >> 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.
> 
> If the check failed, but the page pool assume the page is not from page
> pool and will not do the resource cleaning(like dma unmapping), as the
> page pool still use the page with pfmemalloc set and dma map the page
> if pp_flags & PP_FLAG_DMA_MAP is true in __page_pool_alloc_pages_slow().
> 
> The returning the emergency allocation you mentioned seems to be handled
> in __page_pool_put_page(), see:
> 
> https://elixir.bootlin.com/linux/latest/source/net/core/page_pool.c#L411
> 
> We just use the page with pfmemalloc one time and do the resource cleaning
> before returning the page back to page allocator. Or did I miss something
> here?
> 
> > .
> > 

I think you are right here.  What happens is that the original
pp->signature is OR'ed after the allocation in order to preserve any
existing bits.  When those are present though the if which will trigger the
recycling will fail and those DMA mapping will be left stale.

If we mask the bits during the check (as your patch does), we'll end up not
recycling the page anyway since it has the pfmemalloc bit set. The page
pool recycle function will end up releasing the page and the DMA mappings right?

Regards
/Ilias