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



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