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. > 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.
