Re: [PATCH 1/3] hugetlb: fix vma lock handling during split vma and range unmapping

[Miaohe Lin]

On 2022/10/18 10:56, Mike Kravetz wrote:
> On 10/15/22 09:25, Miaohe Lin wrote:
>> Sorry for late respond. It's a really busy week. :)
>>
>> On 2022/10/5 9:17, Mike Kravetz wrote:
>>> The hugetlb vma lock hangs off the vm_private_data field and is specific
>>> to the vma.  When vm_area_dup() is called as part of vma splitting,  the
>>
>> Oh, I checked vm_area_dup() from callsite of copy_vma and dup_mmap but split_vma
>> is missed... And yes, vma splitting can occur but vma merging won't for hugetlb
>> vma. Thanks for catching this, Mike.
>>
>>> vma lock pointer is copied to the new vma.  This will result in issues
>>> such as double freeing of the structure.  Update the hugetlb open vm_ops
>>> to allocate a new vma lock for the new vma.
>>>
>>> The routine __unmap_hugepage_range_final unconditionally unset
>>> VM_MAYSHARE to prevent subsequent pmd sharing.  hugetlb_vma_lock_free
>>> attempted to anticipate this by checking both VM_MAYSHARE and VM_SHARED.
>>> However, if only VM_MAYSHARE was set we would miss the free.  With the
>>> introduction of the vma lock, a vma can not participate in pmd sharing
>>> if vm_private_data is NULL.  Instead of clearing VM_MAYSHARE in
>>> __unmap_hugepage_range_final, free the vma lock to prevent sharing.  Also,
>>> update the sharing code to make sure vma lock is indeed a condition for
>>> pmd sharing.  hugetlb_vma_lock_free can then key off VM_MAYSHARE and not
>>> miss any vmas.
>>>
>>> Fixes: "hugetlb: add vma based lock for pmd sharing"
>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>> ---
>>>  mm/hugetlb.c | 43 +++++++++++++++++++++++++++----------------
>>>  mm/memory.c  |  4 ----
>>>  2 files changed, 27 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 4443e87e814b..0129d371800c 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -4612,7 +4612,14 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
>>>  		kref_get(&resv->refs);
>>>  	}
>>>  
>>> -	hugetlb_vma_lock_alloc(vma);
>>> +	/*
>>> +	 * vma_lock structure for sharable mappings is vma specific.
>>> +	 * Clear old pointer (if copied via vm_area_dup) and create new.
>>> +	 */
>>> +	if (vma->vm_flags & VM_MAYSHARE) {
>>> +		vma->vm_private_data = NULL;
>>> +		hugetlb_vma_lock_alloc(vma);
>>> +	}
>>
>> IMHO this would lead to memoryleak. Think about the below move_vma() flow:
>> move_vma
>>   copy_vma
>>     new_vma = vm_area_dup(vma);
>>     new_vma->vm_ops->open(new_vma); --> new_vma has its own vma lock.
>>   is_vm_hugetlb_page(vma)
>>     clear_vma_resv_huge_pages
>>       hugetlb_dup_vma_private --> vma->vm_private_data is set to NULL
>>       				  without put ref. So vma lock is *leaked*?
> 
> You are right, that could lead to a leak.
> 
> I have an idea about setting vma->vm_private_data to NULL for VM_MAYSHARE
> vmas in routines like hugetlb_dup_vma_private().  We can check
> hugetlb_vma_lock->vma and only set to NULL if,
> 
> 	vma->(hugetlb_vma_lock)vma->vm_private_data->vma != vma

Looks feasible. Thanks for your work, Mike.

Thanks,
Miaohe Lin

> 
> Got sidetracked chasing down another leak today.  Will send a patch
> implementing this idea soon.
> 
> Thanks for looking at this!
>