Re: [PATCH v3 04/20] mm: VMA sequence count

[Sergey Senozhatsky]

Hi,

On (09/13/17 18:56), Laurent Dufour wrote:
> Hi Sergey,
> 
> On 13/09/2017 13:53, Sergey Senozhatsky wrote:
> > Hi,
> > 
> > On (09/08/17 20:06), Laurent Dufour wrote:
[..]
> > ok, so what I got on my box is:
> > 
> > vm_munmap()  -> down_write_killable(&mm->mmap_sem)
> >  do_munmap()
> >   __split_vma()
> >    __vma_adjust()  -> write_seqcount_begin(&vma->vm_sequence)
> >                    -> write_seqcount_begin_nested(&next->vm_sequence, SINGLE_DEPTH_NESTING)
> > 
> > so this gives 3 dependencies  ->mmap_sem   ->   ->vm_seq
> >                               ->vm_seq     ->   ->vm_seq/1
> >                               ->mmap_sem   ->   ->vm_seq/1
> > 
> > 
> > SyS_mremap() -> down_write_killable(&current->mm->mmap_sem)
> >  move_vma()   -> write_seqcount_begin(&vma->vm_sequence)
> >               -> write_seqcount_begin_nested(&new_vma->vm_sequence, SINGLE_DEPTH_NESTING);
> >   move_page_tables()
> >    __pte_alloc()
> >     pte_alloc_one()
> >      __alloc_pages_nodemask()
> >       fs_reclaim_acquire()
> > 
> > 
> > I think here we have prepare_alloc_pages() call, that does
> > 
> >         -> fs_reclaim_acquire(gfp_mask)
> >         -> fs_reclaim_release(gfp_mask)
> > 
> > so that adds one more dependency  ->mmap_sem   ->   ->vm_seq    ->   fs_reclaim
> >                                   ->mmap_sem   ->   ->vm_seq/1  ->   fs_reclaim
> > 
> > 
> > now, under memory pressure we hit the slow path and perform direct
> > reclaim. direct reclaim is done under fs_reclaim lock, so we end up
> > with the following call chain
> > 
> > __alloc_pages_nodemask()
> >  __alloc_pages_slowpath()
> >   __perform_reclaim()       ->   fs_reclaim_acquire(gfp_mask);
> >    try_to_free_pages()
> >     shrink_node()
> >      shrink_active_list()
> >       rmap_walk_file()      ->   i_mmap_lock_read(mapping);
> > 
> > 
> > and this break the existing dependency. since we now take the leaf lock
> > (fs_reclaim) first and the the root lock (->mmap_sem).
> 
> Thanks for looking at this.
> I'm sorry, I should have miss something.

no prob :)


> My understanding is that there are 2 chains of locks:
>  1. from __vma_adjust() mmap_sem -> i_mmap_rwsem -> vm_seq
>  2. from move_vmap() mmap_sem -> vm_seq -> fs_reclaim
>  2. from __alloc_pages_nodemask() fs_reclaim -> i_mmap_rwsem

yes, as far as lockdep warning suggests.

> So the solution would be to have in __vma_adjust()
>  mmap_sem -> vm_seq -> i_mmap_rwsem
> 
> But this will raised the following dependency from  unmap_mapping_range()
> unmap_mapping_range() 		-> i_mmap_rwsem
>  unmap_mapping_range_tree()
>   unmap_mapping_range_vma()
>    zap_page_range_single()
>     unmap_single_vma()
>      unmap_page_range()	 	-> vm_seq
> 
> And there is no way to get rid of it easily as in unmap_mapping_range()
> there is no VMA identified yet.
> 
> That's being said I can't see any clear way to get lock dependency cleaned
> here.
> Furthermore, this is not clear to me how a deadlock could happen as vm_seq
> is a sequence lock, and there is no way to get blocked here.

as far as I understand,
   seq locks can deadlock, technically. not on the write() side, but on
the read() side:

read_seqcount_begin()
 raw_read_seqcount_begin()
   __read_seqcount_begin()

and __read_seqcount_begin() spins for ever

   __read_seqcount_begin()
   {
    repeat:
     ret = READ_ONCE(s->sequence);
     if (unlikely(ret & 1)) {
         cpu_relax();
         goto repeat;
     }
     return ret;
   }


so if there are two CPUs, one doing write_seqcount() and the other one
doing read_seqcount() then what can happen is something like this

	CPU0					CPU1

						fs_reclaim_acquire()
	write_seqcount_begin()
	fs_reclaim_acquire()			read_seqcount_begin()
	write_seqcount_end()

CPU0 can't write_seqcount_end() because of fs_reclaim_acquire() from
CPU1, CPU1 can't read_seqcount_begin() because CPU0 did write_seqcount_begin()
and now waits for fs_reclaim_acquire(). makes sense?

	-ss