From: Jann Horn <ja...@google.com> Date: Thu, 14 Feb 2019 22:26:22 +0100
> On Thu, Feb 14, 2019 at 6:13 PM David Miller <da...@davemloft.net> wrote: >> >> From: Jann Horn <ja...@google.com> >> Date: Wed, 13 Feb 2019 22:45:59 +0100 >> >> > 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. >> > >> > 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. >> > >> > Signed-off-by: Jann Horn <ja...@google.com> >> >> Applied and queued up for -stable. > > I had sent a v2 at Alexander Duyck's request an hour before you > applied the patch (with a minor difference that, in Alexander's > opinion, might be slightly more efficient). I guess the net tree > doesn't work like the mm tree, where patches can get removed and > replaced with newer versions? So if Alexander wants that change > (s/size/PAGE_FRAG_CACHE_MAX_SIZE/ in the refcount), someone has to > send that as a separate patch? Yes, please send a follow-up. Sorry about that.
