Re: [PATCHv3 4/5] mm: make compound_head() robust

[Hugh Dickins]

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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