Messages in this thread | | | Date | Wed, 7 Aug 2013 14:00:21 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] aoe: adjust ref of head for compound page tails |
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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.
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