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

[Ed Cashin <ecashin@coraid.com>]

On Aug 7, 2013, at 5:27 PM, Andrew Morton wrote:

> On Wed, 7 Aug 2013 14:18:35 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
> 
>> On Wed, 7 Aug 2013 17:12:36 -0400 Ed Cashin <ecashin@coraid.com> wrote:
>> 
>>> 
>>> On Aug 7, 2013, at 4:58 PM, Andrew Morton wrote:
>>> 
>>>> On Thu, 1 Aug 2013 21:29:59 -0400 Ed Cashin <ecashin@coraid.com> wrote:
>>>> 
>>>>> As discussed previously,
>>>> 
>>>> I think I missed that.
>>>> 
>>>>> 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.
>>>> 
>>>> Eh?  What code is putting count==0 pages into bios?  That sounds very
>>>> weird and broken.
>>> 
>>> I thought so in 2007 but couldn't solicit a clear "this is wrong" consensus from the discussion.
>>> 
>>>  http://article.gmane.org/gmane.linux.kernel/499197
>>>  https://lkml.org/lkml/2007/1/19/56
>>>  https://lkml.org/lkml/2006/12/18/230
>>> 
>>> We were seeing zero-count pages in bios from XFS, but Christoph Hellwig pointed out that kmalloced pages can also come from ext3 when it's doing log recovery, and they'll have zero page counts.
>> 
>> aiiee!
>> 
>> It is (I suppose) reasonable to put kmalloced memory into a BIO's page
>> array.  And it is perfectly reasonable for a user of that bio to do a
>> get_page/put_page against that page.  It is utterly unreasonable for
>> the damn page to get freed as a result!
>> 
>> I'd claim that slab is broken.  The page is in use, so it should have an
>> elevated refcount, full stop.
>> 
> 
> err, no.  slab.c uses alloc_pages(), so the underlying page indeed has
> a proper refcount.  I'm still not understanding how this situation comes
> about.

It sounds like it's wrong to give block pages with a zero count, so why not just have aoe BUG_ON(compound_trans_head(bv->page->_count) == 0) until we're sure nobody does that anymore?

If that idea makes sense to you, I will submit a new patch to follow the one under discussion.

-- 
  Ed Cashin
  ecashin@coraid.com


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