On Thu, Feb 14, 2019 at 11:21 PM David Miller <da...@davemloft.net> wrote: > > 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.
@Alexander Do you want to do that? It was your idea and I don't think I can reasonably judge the usefulness of the change.
