Re: [PATCH v2] hugetlb: fix memory leak associated with vma_lock structure

[Miaohe Lin]

On 2022/10/20 4:19, Mike Kravetz wrote:
> The hugetlb vma_lock structure hangs off the vm_private_data pointer
> of sharable hugetlb vmas.  The structure is vma specific and can not
> be shared between vmas.  At fork and various other times, vmas are
> duplicated via vm_area_dup().  When this happens, the pointer in the
> newly created vma must be cleared and the structure reallocated.  Two
> hugetlb specific routines deal with this hugetlb_dup_vma_private and
> hugetlb_vm_op_open.  Both routines are called for newly created vmas.
> hugetlb_dup_vma_private would always clear the pointer and
> hugetlb_vm_op_open would allocate the new vms_lock structure.  This did
> not work in the case of this calling sequence pointed out in [1].
>   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
> When clearing hugetlb_dup_vma_private we actually leak the associated
> vma_lock structure.
> 
> The vma_lock structure contains a pointer to the associated vma.  This
> information can be used in hugetlb_dup_vma_private and hugetlb_vm_op_open
> to ensure we only clear the vm_private_data of newly created (copied)
> vmas.  In such cases, the vma->vma_lock->vma field will not point to the
> vma.
> 
> Update hugetlb_dup_vma_private and hugetlb_vm_op_open to not clear
> vm_private_data if vma->vma_lock->vma == vma.  Also, log a warning if
> hugetlb_vm_op_open ever encounters the case where vma_lock has already
> been correctly allocated for the vma.
> 
> [1] https://lore.kernel.org/linux-mm/5154292a-4c55-28cd-0935-82441e512fc3@xxxxxxxxxx/
> 
> Fixes: 131a79b474e9 ("hugetlb: fix vma lock handling during split vma and range unmapping")
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Thanks for update. This patch looks good to me.

Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>

Thanks,
Miaohe Lin


> ---
> v2 - Allocate vma_lock in hugetlb_vm_op_open if !vma->vm_private_data
>      on entry.  Thanks Miaohe Lin!
> 
>  mm/hugetlb.c | 35 +++++++++++++++++++++++++++--------
>  1 file changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 02f781624fce..ccdffc2fa1ca 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1014,15 +1014,23 @@ void hugetlb_dup_vma_private(struct vm_area_struct *vma)
>  	VM_BUG_ON_VMA(!is_vm_hugetlb_page(vma), vma);
>  	/*
>  	 * Clear vm_private_data
> +	 * - For shared mappings this is a per-vma semaphore that may be
> +	 *   allocated in a subsequent call to hugetlb_vm_op_open.
> +	 *   Before clearing, make sure pointer is not associated with vma
> +	 *   as this will leak the structure.  This is the case when called
> +	 *   via clear_vma_resv_huge_pages() and hugetlb_vm_op_open has already
> +	 *   been called to allocate a new structure.
>  	 * - For MAP_PRIVATE mappings, this is the reserve map which does
>  	 *   not apply to children.  Faults generated by the children are
>  	 *   not guaranteed to succeed, even if read-only.
> -	 * - For shared mappings this is a per-vma semaphore that may be
> -	 *   allocated in a subsequent call to hugetlb_vm_op_open.
>  	 */
> -	vma->vm_private_data = (void *)0;
> -	if (!(vma->vm_flags & VM_MAYSHARE))
> -		return;
> +	if (vma->vm_flags & VM_MAYSHARE) {
> +		struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
> +
> +		if (vma_lock && vma_lock->vma != vma)
> +			vma->vm_private_data = NULL;
> +	} else
> +		vma->vm_private_data = NULL;
>  }
>  
>  /*
> @@ -4601,6 +4609,7 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
>  	struct resv_map *resv = vma_resv_map(vma);
>  
>  	/*
> +	 * HPAGE_RESV_OWNER indicates a private mapping.
>  	 * This new VMA should share its siblings reservation map if present.
>  	 * The VMA will only ever have a valid reservation map pointer where
>  	 * it is being copied for another still existing VMA.  As that VMA
> @@ -4615,11 +4624,21 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
>  
>  	/*
>  	 * vma_lock structure for sharable mappings is vma specific.
> -	 * Clear old pointer (if copied via vm_area_dup) and create new.
> +	 * Clear old pointer (if copied via vm_area_dup) and allocate
> +	 * new structure.  Before clearing, make sure vma_lock is not
> +	 * for this vma.
>  	 */
>  	if (vma->vm_flags & VM_MAYSHARE) {
> -		vma->vm_private_data = NULL;
> -		hugetlb_vma_lock_alloc(vma);
> +		struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
> +
> +		if (vma_lock) {
> +			if (vma_lock->vma != vma) {
> +				vma->vm_private_data = NULL;
> +				hugetlb_vma_lock_alloc(vma);
> +			} else
> +				pr_warn("HugeTLB: vma_lock already exists in %s.\n", __func__);
> +		} else
> +			hugetlb_vma_lock_alloc(vma);
>  	}
>  }
>  
>