Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 17 Mar 2010 12:00:15 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> On Wed, Mar 17, 2010 at 11:12 AM, KAMEZAWA Hiroyuki
> > BTW, I doubt freeing anon_vma can happen even when we check mapcount.
> >
> > "unmap" is 2-stage operation.
> >        1. unmap_vmas() => modify ptes, free pages, etc.
> >        2. free_pgtables() => free pgtables, unlink vma and free it.
> >
> > Then, if migration is enough slow.
> >
> >        Migration():                            Exit():
> >        check mapcount
> >        rcu_read_lock
> >        pte_lock
> >        replace pte with migration pte
> >        pte_unlock
> >                                                pte_lock
> >        copy page etc...                        zap pte (clear pte)
> >                                                pte_unlock
> >                                                free_pgtables
> >                                                ->free vma
> >                                                ->free anon_vma
> >        pte_lock
> >        remap pte with new pfn(fail)
> >        pte_unlock
> >
> >        lock anon_vma->lock             # modification after free.
> >        check list is empty
> 
> check list is empty?
> Do you mean anon_vma->head?
> 
yes.

> If it is, is it possible that that list isn't empty since anon_vma is
> used by others due to
> SLAB_DESTROY_BY_RCU?
> 
There are 4 cases.
	A) anon_vma->list is not empty because anon_vma is not freed.
	B) anon_vma->list is empty because it's freed.
	C) anon_vma->list is empty but it's reused.
	D) anon_vma->list is not empty but it's reused.
 
> but such case is handled by page_check_address, vma_address, I think.
> 
yes. Then, this corrupt nothing, as I wrote. We just modify anon_vma->lock
and it's safe because of SLAB_DESTROY_BY_RCU.


> >        unlock anon_vma->lock
> >        free anon_vma
> >        rcu_read_unlock
> >
> >
> > Hmm. IIUC, anon_vma is allocated as SLAB_DESTROY_BY_RCU. Then, while
> > rcu_read_lock() is taken, anon_vma is anon_vma even if freed. But it
> > may reused as anon_vma for someone else.
> > (IOW, it may be reused but never pushed back to general purpose memory
> >  until RCU grace period.)
> > Then, touching anon_vma->lock never cause any corruption.
> >
> > Does use-after-free check for SLAB_DESTROY_BY_RCU correct behavior ?
> 
> Could you elaborate your point?
> 

Ah, my point is "how use-after-free is detected ?"

If use-after-free is detected by free_pages() (DEBUG_PGALLOC), it seems
strange because DESTROY_BY_RCU guarantee that never happens.

So, I assume use-after-free is detected in SLAB layer. If so,
in above B), C), D) case, it seems there is use-after free in slab's point
of view but it works as expected, no corruption.

Then, my question is
"Does use-after-free check for SLAB_DESTROY_BY_RCU work correctly ?"

and implies we need this patch ?
(But this will prevent unnecessary page copy etc. by easy check.)

Thanks,
-Kame