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.

Reply via email to