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

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Date:   Thu, 22 Jun 2017 21:36:46 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     willemdebruijn.kernel@...il.com
Cc:     netdev@...r.kernel.org, linux-api@...r.kernel.org,
        willemb@...gle.com
Subject: Re: [PATCH net-next v3 02/13] sock: skb_copy_ubufs support for
 compound pages

From: Willem de Bruijn <willemdebruijn.kernel@...il.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.