netdev - Re: [PATCH 1/4 net-next] net: allow skb->head to be a page fragment

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Date:	Wed, 07 Nov 2012 03:04:24 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Li Yu <raise.sail@...il.com>
Cc:	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/4 net-next] net: allow skb->head to be a page fragment

On Wed, 2012-11-07 at 16:35 +0800, Li Yu wrote:
> 于 2012年11月07日 16:20, Li Yu 写道:
> > 于 2012年04月27日 18:33, Eric Dumazet 写道:
> >> From: Eric Dumazet <edumazet@...gle.com>
> >>
> >> skb->head is currently allocated from kmalloc(). This is convenient but
> >> has the drawback the data cannot be converted to a page fragment if
> >> needed.
> >>
> >
> > Hi, Eric,
> >
> > I have a question about this patch, why data are allocated from
> > kmalloc() can not be converted to page fragment ? We have its kernel
> > mapped address and length, so we can get its page and offset in the
> > page. If the skb is not cloned (shared with others), such page and its
> > offset should be can use safely, in my words.
> >
> > I suspected that I may lost important something in slab internals, is
> > right?
> >
> > Thanks
> >
> > Yu
> >
> 
> Or since slab allocated object may be across multiple pages?
> I saw __netdev_alloc_skb() only use build_skb() with non-zero
> fragsz if it is less than PAGE_SIZE.
> 

SLAB allocators have their own way to handle a page, we cant interfere
with it.

So no, a kmalloced() object cannot be converted to a page fragment.



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