netdev - Re: [PATCH net-next v3 02/13] sock: skb_copy

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Message-ID: <CAF=yD-+WQ1b9BytG4JuhgHCR8gAKvUVJPUfi4t-u_DqAFnVrkA@mail.gmail.com>
Date:   Thu, 22 Jun 2017 16:57:07 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Network Development <netdev@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v3 02/13] sock: skb_copy_ubufs support for
 compound pages

>
> 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.