Re: [PATCH v2 2/7] mm: Extend copy_vma()

[Kirill Tkhai]

Hi, Kirill,

On 21.05.2019 11:18, Kirill A. Shutemov wrote:
> On Mon, May 20, 2019 at 05:00:12PM +0300, Kirill Tkhai wrote:
>> This prepares the function to copy a vma between
>> two processes. Two new arguments are introduced.
> 
> This kind of changes requires a lot more explanation in commit message,
> describing all possible corner cases> For instance, I would really like to see a story on why logic around
> need_rmap_locks is safe after the change.

Let me fast answer on the below question firstly, and later I'll write
wide explanations, since this requires much more time.
 
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
>> ---
>>  include/linux/mm.h |    4 ++--
>>  mm/mmap.c          |   33 ++++++++++++++++++++++++---------
>>  mm/mremap.c        |    4 ++--
>>  3 files changed, 28 insertions(+), 13 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 0e8834ac32b7..afe07e4a76f8 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2329,8 +2329,8 @@ extern void __vma_link_rb(struct mm_struct *, struct vm_area_struct *,
>>  	struct rb_node **, struct rb_node *);
>>  extern void unlink_file_vma(struct vm_area_struct *);
>>  extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
>> -	unsigned long addr, unsigned long len, pgoff_t pgoff,
>> -	bool *need_rmap_locks);
>> +	struct mm_struct *, unsigned long addr, unsigned long len,
>> +	pgoff_t pgoff, bool *need_rmap_locks, bool clear_flags_ctx);
>>  extern void exit_mmap(struct mm_struct *);
>>  
>>  static inline int check_data_rlimit(unsigned long rlim,
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 57803a0a3a5c..99778e724ad1 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -3195,19 +3195,21 @@ int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
>>  }
>>  
>>  /*
>> - * Copy the vma structure to a new location in the same mm,
>> - * prior to moving page table entries, to effect an mremap move.
>> + * Copy the vma structure to new location in the same vma
>> + * prior to moving page table entries, to effect an mremap move;
>>   */
>>  struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>> -	unsigned long addr, unsigned long len, pgoff_t pgoff,
>> -	bool *need_rmap_locks)
>> +				struct mm_struct *mm, unsigned long addr,
>> +				unsigned long len, pgoff_t pgoff,
>> +				bool *need_rmap_locks, bool clear_flags_ctx)
>>  {
>>  	struct vm_area_struct *vma = *vmap;
>>  	unsigned long vma_start = vma->vm_start;
>> -	struct mm_struct *mm = vma->vm_mm;
>> +	struct vm_userfaultfd_ctx uctx;
>>  	struct vm_area_struct *new_vma, *prev;
>>  	struct rb_node **rb_link, *rb_parent;
>>  	bool faulted_in_anon_vma = true;
>> +	unsigned long flags;
>>  
>>  	/*
>>  	 * If anonymous vma has not yet been faulted, update new pgoff
>> @@ -3220,15 +3222,25 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>>  
>>  	if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent))
>>  		return NULL;	/* should never get here */
>> -	new_vma = vma_merge(mm, prev, addr, addr + len, vma->vm_flags,
>> -			    vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
>> -			    vma->vm_userfaultfd_ctx);
>> +
>> +	uctx = vma->vm_userfaultfd_ctx;
>> +	flags = vma->vm_flags;
>> +	if (clear_flags_ctx) {
>> +		uctx = NULL_VM_UFFD_CTX;
>> +		flags &= ~(VM_UFFD_MISSING | VM_UFFD_WP | VM_MERGEABLE |
>> +			   VM_LOCKED | VM_LOCKONFAULT | VM_WIPEONFORK |
>> +			   VM_DONTCOPY);
>> +	}
> 
> Why is the new logic required? No justification given.

Ditto.

>> +
>> +	new_vma = vma_merge(mm, prev, addr, addr + len, flags, vma->anon_vma,
>> +			    vma->vm_file, pgoff, vma_policy(vma), uctx);
>>  	if (new_vma) {
>>  		/*
>>  		 * Source vma may have been merged into new_vma
>>  		 */
>>  		if (unlikely(vma_start >= new_vma->vm_start &&
>> -			     vma_start < new_vma->vm_end)) {
>> +			     vma_start < new_vma->vm_end) &&
>> +			     vma->vm_mm == mm) {
> 
> How can vma_merge() succeed if vma->vm_mm != mm?

We don't use vma as an argument of vma_merge(). We use vma as a source of
vma->anon_vma, vma->vm_file and vma_policy().

We search some new_vma in mm with the same characteristics as vma has in vma->vm_mm.
In case of success vma_merge() returns it for us. For example, it may success, when
vma->vm_mm is mm_struct of forked process, while mm is mm_struct of its parent.

[...]

Kirill