Re: [PATCH] mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 13 Feb 2019 21:41:57 +0100 Jann Horn <jannh@google.com> wrote:

> The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum
> number of references that we might need to create in the fastpath later,
> the bump-allocation fastpath only has to modify the non-atomic bias value
> that tracks the number of extra references we hold instead of the atomic
> refcount. The maximum number of allocations we can serve (under the
> assumption that no allocation is made with size 0) is nc->size, so that's
> the bias used.
> 
> However, even when all memory in the allocation has been given away, a
> reference to the page is still held; and in the `offset < 0` slowpath, the
> page may be reused if everyone else has dropped their references.
> This means that the necessary number of references is actually
> `nc->size+1`.
> 
> Luckily, from a quick grep, it looks like the only path that can call
> page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which
> requires CAP_NET_ADMIN in the init namespace and is only intended to be
> used for kernel testing and fuzzing.

For the net-naive, what is TAP?  It doesn't appear to mean
drivers/net/tap.c.

> To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the
> `offset < 0` path, below the virt_to_page() call, and then repeatedly call
> writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI,
> with a vector consisting of 15 elements containing 1 byte each.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4675,11 +4675,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
>  		/* Even if we own the page, we do not use atomic_set().
>  		 * This would break get_page_unless_zero() users.
>  		 */
> -		page_ref_add(page, size - 1);
> +		page_ref_add(page, size);
>  
>  		/* reset page count bias and offset to start of new frag */
>  		nc->pfmemalloc = page_is_pfmemalloc(page);
> -		nc->pagecnt_bias = size;
> +		nc->pagecnt_bias = size + 1;
>  		nc->offset = size;
>  	}
>  
> @@ -4695,10 +4695,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
>  		size = nc->size;
>  #endif
>  		/* OK, page count is 0, we can safely set it */
> -		set_page_count(page, size);
> +		set_page_count(page, size + 1);
>  
>  		/* reset page count bias and offset to start of new frag */
> -		nc->pagecnt_bias = size;
> +		nc->pagecnt_bias = size + 1;
>  		offset = size - fragsz;
>  	}

This is probably more a davem patch than a -mm one.