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?
