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

[Alexander Duyck]

On Wed, Feb 13, 2019 at 12:42 PM Jann Horn <jannh@xxxxxxxxxx> 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.

Actually that has me somewhat concerned. I wouldn't be surprised if
most drivers expect the netdev_alloc_frags call to at least output an
SKB_DATA_ALIGN sized value.

We probably should update __netdev_alloc_frag and __napi_alloc_frag so
that they will pass fragsz through SKB_DATA_ALIGN.

> 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.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
>  mm/page_alloc.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 35fdde041f5c..46285d28e43b 100644
> --- 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;
>         }

If we already have to add a constant it might be better to just use
PAGE_FRAG_CACHE_MAX_SIZE + 1 in all these spots where you are having
to use "size + 1" instead of "size". That way we can avoid having to
add a constant to a register value and then program that value.
instead we can just assign the constant value right from the start.