Re: [PATCH net-next v3 02/13] sock: skb_copy_ubufs support for compound pages

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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.

This does not work. Some callers, such as __skb_checksum, cannot
fail, so neither can kmap_compound. Also, vaddr of consecutive
kmap_atomic calls are not guaranteed to be in order. Indeed, on x86
and arm vaddr appears to grows down: (FIXADDR_TOP - ((x) << PAGE_SHIFT))

An alternative is to change the kmap_atomic callers in skbuff.c. To
avoid open coding, we can wrap the kmap_atomic; op; kunmap_atomic
in a macro that loops only if needed:

static inline bool skb_frag_must_loop(struct page *p)
{
#if defined(CONFIG_HIGHMEM) || defined(CONFIG_X86_32)
        if (PageHighMem(p))
                return true;
#endif
        return false;
}

#define skb_frag_map_foreach(f, start, size, p, p_off, cp, copied)      \
        for (p = skb_frag_page(f) + ((start) >> PAGE_SHIFT),            \
             p_off = (start) & (PAGE_SIZE - 1),                         \
             copied = 0,                                                \
             cp = skb_frag_must_loop(p) ?                               \
                    min_t(u32, size, PAGE_SIZE - p_off) : size;         \
             copied < size;                                             \
             copied += cp, p++, p_off = 0,                              \
             cp = min_t(u32, size - copied, PAGE_SIZE))                 \

This does not change behavior on machines without high mem
or on low mem pages.

skb_seq_read keeps a mapping between calls to the function,
so will need a separate approach.