Messages in this thread | | | Date | Thu, 27 Aug 2015 10:28:48 -0700 (PDT) | From | Hugh Dickins <> | Subject | Re: [PATCHv3 4/5] mm: make compound_head() robust |
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On Thu, 27 Aug 2015, Michal Hocko wrote: > On Thu 27-08-15 17:09:17, Michal Hocko wrote: > [...] > > Btw. Do we need the same think for page::mapping and KSM? > > I guess we are safe here because the address for mappings comes from > kmalloc and that aligned properly, right?
Not quite right, in fact. Because usually the struct address_space is embedded within the struct inode (at i_data), and the struct inode embedded within the fs-dependent inode, and that's what's kmalloc'ed.
What makes the mapping pointer low bits safe is include/linux/fs.h: struct address_space { ... } __attribute__((aligned(sizeof(long))));
Which we first had to add in for the cris architecture, which stumbled not on a genuine allocated address_space, but on that funny statically declared swapper_space in mm/swap_state.c.
But struct anon_vma and KSM's struct stable_node (which depend on the same scheme for low bits of page->mapping) have no such alignment attribute specified: those ones are indeed relying on the kmalloc guarantee as you suppose.
Does struct rcu_head have no __attribute__((aligned(whatever)))? Perhaps that attribute should be added when it's needed.
Hugh
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