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

[Michal Hocko <mhocko@kernel.org>]

On Thu 27-08-15 10:28:48, Hugh Dickins wrote:
> 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))));

Oh, right you are.

> 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.

Thanks for the clarification.

> 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.

That's basically what I meant in the previous email.
-- 
Michal Hocko
SUSE Labs

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