On Wed, 7 Aug 2013 14:18:35 -0700 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 7 Aug 2013 17:12:36 -0400 Ed Cashin <ecashin@xxxxxxxxxx> wrote: > > > > > On Aug 7, 2013, at 4:58 PM, Andrew Morton wrote: > > > > > On Thu, 1 Aug 2013 21:29:59 -0400 Ed Cashin <ecashin@xxxxxxxxxx> 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>