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

On 13/09/2017 13:53, Sergey Senozhatsky wrote:
> Hi,
>
> On (09/08/17 20:06), Laurent Dufour wrote:
> [..]
>> @@ -903,6 +910,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>> mm->map_count--;
>> mpol_put(vma_policy(next));
>> kmem_cache_free(vm_area_cachep, next);
>> + write_seqcount_end(&next->vm_sequence);
>> /*
>> * In mprotect's case 6 (see comments on vma_merge),
>> * we must remove another next too. It would clutter
>> @@ -932,11 +940,14 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>> if (remove_next == 2) {
>> remove_next = 1;
>> end = next->vm_end;
>> + write_seqcount_end(&vma->vm_sequence);
>> goto again;
>> - }
>> - else if (next)
>> + } else if (next) {
>> + if (next != vma)
>> + write_seqcount_begin_nested(&next->vm_sequence,
>> + SINGLE_DEPTH_NESTING);
>> vma_gap_update(next);
>> - else {
>> + } else {
>> /*
>> * If remove_next == 2 we obviously can't
>> * reach this path.
>> @@ -962,6 +973,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>> if (insert && file)
>> uprobe_mmap(insert);
>>
>> + if (next && next != vma)
>> + write_seqcount_end(&next->vm_sequence);
>> + write_seqcount_end(&vma->vm_sequence);
>
>
> 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.

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

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.

Cheers,
Laurent.

>
> well, seems to be the case.
>
> -ss
>

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