On Thu, Jun 22, 2017 at 9:36 PM, David Miller <da...@davemloft.net> wrote: > From: Willem de Bruijn <willemdebruijn.ker...@gmail.com> > Date: Thu, 22 Jun 2017 16:57:07 -0400 > >>> >>> Likewise. >>> >>>> + f_off = f->page_offset; >>>> + f_size = f->size; >>>> + >>>> + vaddr = kmap_atomic(skb_frag_page(f)); >>> >>> I looked at some kmap_atomic() implementations and I do not think >>> it supports compound pages. >> >> Indeed. Thanks. It appears that I can do the obvious thing and >> kmap the individual page that is being copied inside the loop: >> >> kmap_atomic(skb_frag_page(f) + (f_off >> PAGE_SHIFT)); >> >> This is similar to existing logic in copy_huge_page_from_user >> and __flush_dcache_page in arch/arm/mm/flush.c >> >> But, this also applies to other skb operations that call kmap_atomic, >> such as skb_copy_bits and __skb_checksum. Not all can be called >> from a codepath with a compound user page, but I have to address >> the ones that can. > > Yeah that's quite a mess, it looks like this assumption that > kmap can handle compound pages exists in quite a few places.
I hadn't even considered that skbs can already hold compound page frags without zerocopy. Open coding all call sites to iterate is tedious and unnecessary in the common case where a page is not highmem. kmap_atomic has enough slots to map an entire order-3 compound page at once. But kmap_atomic cannot fail and there may be edge cases that are larger than order-3. Packet rings allocate with __GFP_COMP and an order derived from (user supplied) tp_block_size, for instance. But it links each skb_frag_t from an individual page, so this case seems okay. Perhaps calls to kmap_atomic can be replaced with a kmap_compound(..) that checks __this_cpu_read(__kmap_atomic_idx) + (1 << compound_order(p)) < KM_TYPE_NR before calling kmap_atomic on all pages in the compound page. In the common case that the page is not high mem, a single call is enough, as there is no per-page operation.
