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

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