Re: [PATCH v2] mm/hugetlb: fix race when migrate pages

[Michal Hocko <mhocko@suse.cz>]

On Wed 20-07-16 18:03:40, zhong jiang wrote:
> On 2016/7/20 15:38, Michal Hocko wrote:
> > [CC Mike and Naoya]
> > On Tue 19-07-16 21:45:58, zhongjiang wrote:
> >> From: zhong jiang <zhongjiang@huawei.com>
> >>
> >> I hit the following code in huge_pte_alloc when run the database and
> >> online-offline memory in the system.
> >>
> >> BUG_ON(pte && !pte_none(*pte) && !pte_huge(*pte));
> >>
> >> when pmd share function enable, we may be obtain a shared pmd entry.
> >> due to ongoing offline memory , the pmd entry points to the page will
> >> turn into migrate condition. therefore, the bug will come up.
> >>
> >> The patch fix it by checking the pmd entry when we obtain the lock.
> >> if the shared pmd entry points to page is under migration. we should
> >> allocate a new pmd entry.
> >
> > I am still not 100% sure this is correct. Does huge_pte_lockptr work
> > properly for the migration swapentry?

What about this part?

> > If yes and we populate the pud
> > with a migration entry then is it really bad/harmful (other than hitting
> > the BUG_ON which might be update to handle that case)? This might be a
> > stupid question, sorry about that, but I have really problem to grasp
> > the whole issue properly and the changelog didn't help me much. I would
> > really appreciate some clarification here. The pmd sharing code is clear
> > as mud and adding new tweaks there doesn't sound like it would make it
> > more clear.
>
>     ok, Maybe the following explain will better.
>     cpu0                                                                      cpu1
>     try_to_unmap_one                                 huge_pmd_share                             
>         page_check_address                                 huge_pte_lockptr
>                       spin_lock                                                        
>        (page entry can be set to migrate or
>        Posion )      
>  
>          pte_unmap_unlock
>                                                                             spin_lock
>                                                                             (page entry have changed)

Well, but this is just one possible race. We can also race after
pud_populate:

cpu0				cpu1
try_to_unmap_on			huge_pmd_share
  page_check_address		  huge_pte_lockptr
  				    spin_lock
				    pud_populate
				    spin_unlock
  spin_lock(ptl)
  set_migration_entry
  spun_unlock()
  				  pmd_alloc # we will get migration entry

So unless I am missing something here we either have to be able to cope
with migration entries somehow already or we need some additional care
for shared pmd entries (aka shared pud page) for this to behave
properly.
-- 
Michal Hocko
SUSE Labs

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