On Wed, Feb 13, 2019 at 11:42 PM Alexander Duyck <alexander.du...@gmail.com> wrote: > On Wed, Feb 13, 2019 at 12:42 PM Jann Horn <ja...@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. > > 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.
Do you want to do a separate patch for that? I'd like to not mix logically separate changes in a single patch, and I also don't have a good understanding of the alignment concerns here. > > 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: sta...@vger.kernel.org > > Signed-off-by: Jann Horn <ja...@google.com> > > --- > > 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. I doubt that these few instructions make a difference, but sure, I can send a v2 with that changed.
