Re: [PATCH v9 07/24] mm: VMA sequence count

[Laurent Dufour]

On 27/03/2018 23:30, David Rientjes wrote:
> On Tue, 13 Mar 2018, Laurent Dufour wrote:
> 
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index faf85699f1a1..5898255d0aeb 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -558,6 +558,10 @@ void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
>>  	else
>>  		mm->highest_vm_end = vm_end_gap(vma);
>>  
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +	seqcount_init(&vma->vm_sequence);
>> +#endif
>> +
>>  	/*
>>  	 * vma->vm_prev wasn't known when we followed the rbtree to find the
>>  	 * correct insertion point for that vma. As a result, we could not
>> @@ -692,6 +696,30 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>  	long adjust_next = 0;
>>  	int remove_next = 0;
>>  
>> +	/*
>> +	 * Why using vm_raw_write*() functions here to avoid lockdep's warning ?
>> +	 *
>> +	 * Locked is complaining about a theoretical lock dependency, involving
>> +	 * 3 locks:
>> +	 *   mapping->i_mmap_rwsem --> vma->vm_sequence --> fs_reclaim
>> +	 *
>> +	 * Here are the major path leading to this dependency :
>> +	 *  1. __vma_adjust() mmap_sem  -> vm_sequence -> i_mmap_rwsem
>> +	 *  2. move_vmap() mmap_sem -> vm_sequence -> fs_reclaim
>> +	 *  3. __alloc_pages_nodemask() fs_reclaim -> i_mmap_rwsem
>> +	 *  4. unmap_mapping_range() i_mmap_rwsem -> vm_sequence
>> +	 *
>> +	 * So there is no way to solve this easily, especially because in
>> +	 * unmap_mapping_range() the i_mmap_rwsem is grab while the impacted
>> +	 * VMAs are not yet known.
>> +	 * However, the way the vm_seq is used is guarantying that we will
>> +	 * never block on it since we just check for its value and never wait
>> +	 * for it to move, see vma_has_changed() and handle_speculative_fault().
>> +	 */
>> +	vm_raw_write_begin(vma);
>> +	if (next)
>> +		vm_raw_write_begin(next);
>> +
>>  	if (next && !insert) {
>>  		struct vm_area_struct *exporter = NULL, *importer = NULL;
>>  
> 
> Eek, what about later on:
> 
> 		/*
> 		 * Easily overlooked: when mprotect shifts the boundary,
> 		 * make sure the expanding vma has anon_vma set if the
> 		 * shrinking vma had, to cover any anon pages imported.
> 		 */
> 		if (exporter && exporter->anon_vma && !importer->anon_vma) {
> 			int error;
> 
> 			importer->anon_vma = exporter->anon_vma;
> 			error = anon_vma_clone(importer, exporter);
> 			if (error)
> 				return error;
> 		}
> 
> This needs
> 
> if (error) {
> 	if (next && next != vma)
> 		vm_raw_write_end(next);
> 	vm_raw_write_end(vma);
> 	return error;
> }

Nice catch !

Thanks,
Laurent.