Le lundi 28 novembre 2011 à 10:06 -0800, Alexander Duyck a écrit :
> On 11/27/2011 12:07 AM, Eric Dumazet wrote:
> > But why are these drivers using kmap_atomic() in first place, since
> > their fragments are allocated in regular zone (GFP_ATOMIC or
> > GFP_KERNEL) ?
>
> I was asking the same thing myself recently when I started working on
> some copy-break like code for the ixgbe driver. I believe the main
> reason is a lack of documentation. This code is based loosely on the
> skb_copy_bits code which will use kmap_skb_frag over all of the paged
> portions of the sk_buff. As such it was decided to map things via
> kmap_atomic in order to guarantee the pages had a valid virtual address.
>
> If I understand things correctly, what you are brining up is that pages
> allocated with either GFP_ATOMIC or GFP_KERNEL will always be allocated
> from the lowmem pool and as such page_address should always succeed. Is
> that correct?
>
Yes.
Either you could :
A) allocate high memory pages to lower pressure on LOWMEM area on 32bit
kernels.
B) avoid using kmap_atomic() & kunmap_atomic() and be faster on 32bit
kernels.
skb_put(skb, length);
memcpy(skb->data, page_address(buffer_info->page), length);
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