Re: Multiple potential races on vma->vm_flags

[Hugh Dickins]

On Fri, 11 Sep 2015, Kirill A. Shutemov wrote:
> On Thu, Sep 10, 2015 at 03:27:59PM +0200, Andrey Konovalov wrote:
> > Can a vma be shared among a few mm's?
> 
> Define "shared".
> 
> vma can belong only to one process (mm_struct), but it can be accessed
> from other process like in rmap case below.
> 
> rmap uses anon_vma_lock for anon vma and i_mmap_rwsem for file vma to make
> sure that the vma will not disappear under it.
> 
> > If yes, then taking current->mm->mmap_sem to protect vma is not enough.
> 
> Depends on what protection you are talking about.
>  
> > In the first report below both T378 and T398 take
> > current->mm->mmap_sem at mm/mlock.c:650, but they turn out to be
> > different locks (the addresses are different).
> 
> See i_mmap_lock_read() in T398. It will guarantee that vma is there.
> 
> > In the second report T309 doesn't take any locks at all, since it
> > assumes that after checking atomic_dec_and_test(&mm->mm_users) the mm
> > has no other users, but then it does a write to vma.
> 
> This one is tricky. I *assume* the mm cannot be generally accessible after
> mm_users drops to zero, but I'm not entirely sure about it.
> procfs? ptrace?

Most of the things (including procfs and ptrace) that need to work on
a foreign mm do take a hold on mm_users with get_task_mm().  swapoff
uses atomic_inc_not_zero(&mm->mm_users).  In KSM I managed to get away
with just a hold on the structure itself, atomic_inc(&mm->mm_count),
and a check for mm_users 0 wherever it down_reads mmap_sem (but Andrey
might like to turn KSM on: it wouldn't be entirely shocking if he were
to discover an anomaly from that).

> 
> The VMA is still accessible via rmap at this point. And I think it can be
> a problem:
> 
> 		CPU0					CPU1
> exit_mmap()
>   // mmap_sem is *not* taken
>   munlock_vma_pages_all()
>     munlock_vma_pages_range()
>     						try_to_unmap_one()
> 						  down_read_trylock(&vma->vm_mm->mmap_sem))
> 						  !!(vma->vm_flags & VM_LOCKED) == true
>       vma->vm_flags &= ~VM_LOCKED;
>       <munlock the page>
>       						  mlock_vma_page(page);
> 						  // mlocked pages is leaked.
> 
> The obvious solution is to take mmap_sem in exit path, but it would cause
> performance regression.
> 
> Any comments?

I'm inclined to echo Vlastimil's comment from earlier in the thread:
sounds like an overkill, unless we find something more serious than this.

I'm not sure whether we'd actually see a regression from taking mmap_sem
in exit path; but given that it's mmap_sem, yes, history tells us please
not to take it any more than we have to.

I do remember wishing, when working out KSM's mm handling, that exit took
mmap_sem: it would have made it simpler, but that wasn't a change I dared
to make.

Maybe an mm_users 0 check after down_read_trylock in try_to_unmap_one() 
could fix it?

But if we were to make a bigger change for this VM_LOCKED issue, and
something more serious makes it worth all the effort, I'd say that
what needs to be done is to give mlock/munlock proper locking (haha).

I have not yet looked at your mlocked THP patch (sorry), but when I
was doing the same thing for huge tmpfs, what made it so surprisingly
difficult was all the spongy trylocking, which concealed the rules.

Maybe I'm completely wrong, but I thought a lot of awkwardness might
disappear if they were relying on anon_vma->rwsem and i_mmap_rwsem
throughout instead of mmap_sem.

Hugh
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