Re: [PATCH] aoe: adjust ref of head for compound page tails

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 1 Aug 2013 21:29:59 -0400 Ed Cashin <ecashin@coraid.com> wrote:

> As discussed previously, the fact that some users of the block
> layer provide bios that point to pages with a zero _count means
> that it is not OK for the network layer to do a put_page on the
> skb frags during an skb_linearize, so the aoe driver gets a
> reference to pages in bios and puts the reference before ending
> the bio.  And because it cannot use get_page on a page with a
> zero _count, it manipulates the value directly.
> 
> It is not OK to increment the _count of a compound page tail,
> though, since the VM layer will VM_BUG_ON a non-zero _count.
> Block users that do direct I/O can result in the aoe driver
> seeing compound page tails in bios.  In that case, the same logic
> works as long as the head of the compound page is used instead of
> the tails.  This patch handles compound pages and does not BUG.
> 
> It relies on the block layer user leaving the relationship
> between the page tail and its head alone for the duration between
> the submission of the bio and its completion, whether successful
> or not.

What are the end-user-visible effects of the problem which is being
fixed?  Please always include this info when fixing bugs so that others
can work out what kernel version(s) need the patch.