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

[Minchan Kim]

On Wed, Mar 17, 2010 at 12:15 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Wed, 17 Mar 2010 12:00:15 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> 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.
>> > Â Â Â Â1. unmap_vmas() => modify ptes, free pages, etc.
>> > Â Â Â Â2. free_pgtables() => free pgtables, unlink vma and free it.
>> >
>> > Then, if migration is enough slow.
>> >
>> > Â Â Â ÂMigration(): Â Â Â Â Â Â Â Â Â Â Â Â Â ÂExit():
>> > Â Â Â Âcheck mapcount
>> > Â Â Â Ârcu_read_lock
>> > Â Â Â Âpte_lock
>> > Â Â Â Âreplace pte with migration pte
>> > Â Â Â Âpte_unlock
>> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âpte_lock
>> > Â Â Â Âcopy page etc... Â Â Â Â Â Â Â Â Â Â Â Âzap pte (clear pte)
>> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âpte_unlock
>> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âfree_pgtables
>> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â->free vma
>> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â->free anon_vma
>> > Â Â Â Âpte_lock
>> > Â Â Â Âremap pte with new pfn(fail)
>> > Â Â Â Âpte_unlock
>> >
>> >    Âlock anon_vma->lock       # modification after free.
>> > Â Â Â Âcheck 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.
> Â Â Â ÂA) anon_vma->list is not empty because anon_vma is not freed.
> Â Â Â ÂB) anon_vma->list is empty because it's freed.
> Â Â Â ÂC) anon_vma->list is empty but it's reused.
> Â Â Â ÂD) anon_vma->list is not empty but it's reused.

E) anon_vma is used for other object.

That's because we don't hold rcu_read_lock.
I think Mel met this E) situation.

AFAIU, even slab page of SLAB_BY_RCU can be freed after grace period.
Do I miss something?

>
>> 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.
>
>
>> > Â Â Â Âunlock anon_vma->lock
>> > Â Â Â Âfree anon_vma
>> > Â Â Â Ârcu_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
>> > Âuntil 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 ?"
>

I am not sure Mel found that by DEBUG_PGALLOC.
But, E) case can be founded by DEBUG_PGALLOC.

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



-- 
Kind regards,
Minchan Kim
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