Re: [PATCH 6/8] hugetlb: add vma based lock for pmd sharing

[Mike Kravetz <mike.kravetz@oracle.com>]

On 08/27/22 17:30, Miaohe Lin wrote:
> On 2022/8/25 1:57, Mike Kravetz wrote:
> > Allocate a rw semaphore and hang off vm_private_data for
> > synchronization use by vmas that could be involved in pmd sharing.  Only
> > add infrastructure for the new lock here.  Actual use will be added in
> > subsequent patch.
> > 
> > Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
> <snip>
> 
> > +static void hugetlb_vma_lock_free(struct vm_area_struct *vma)
> > +{
> > +	/*
> > +	 * Only present in sharable vmas.  See comment in
> > +	 * __unmap_hugepage_range_final about the neeed to check both
> 
> s/neeed/need/
> 
> > +	 * VM_SHARED and VM_MAYSHARE in free path
> 
> I think there might be some wrong checks around this patch. As above comment said, we
> need to check both flags, so we should do something like below instead?
> 
> 	if (!(vma->vm_flags & (VM_MAYSHARE | VM_SHARED) == (VM_MAYSHARE | VM_SHARED)))
> 
> > +	 */

Thanks.  I will update.

> > +	if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
> > +		return;
> > +
> > +	if (vma->vm_private_data) {
> > +		kfree(vma->vm_private_data);
> > +		vma->vm_private_data = NULL;
> > +	}
> > +}
> > +
> > +static void hugetlb_vma_lock_alloc(struct vm_area_struct *vma)
> > +{
> > +	struct rw_semaphore *vma_sema;
> > +
> > +	/* Only establish in (flags) sharable vmas */
> > +	if (!vma || !(vma->vm_flags & VM_MAYSHARE))
> > +		return;
> > +
> > +	/* Should never get here with non-NULL vm_private_data */
> 
> We can get here with non-NULL vm_private_data when called from hugetlb_vm_op_open during fork?

Right!

In fork, We allocate a new semaphore in hugetlb_dup_vma_private, and then
shortly after call hugetlb_vm_op_open.

It works as is, and I can update the comment.  However, I wonder if we should
just clear vm_private_data in hugetlb_dup_vma_private and let hugetlb_vm_op_open
do the allocation.

> 
> Also there's one missing change on comment:
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index d0617d64d718..4bc844a1d312 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -863,7 +863,7 @@ __weak unsigned long vma_mmu_pagesize(struct vm_area_struct *vma)
>   * faults in a MAP_PRIVATE mapping. Only the process that called mmap()
>   * is guaranteed to have their future faults succeed.
>   *
> - * With the exception of reset_vma_resv_huge_pages() which is called at fork(),
> + * With the exception of hugetlb_dup_vma_private() which is called at fork(),
>   * the reserve counters are updated with the hugetlb_lock held. It is safe
>   * to reset the VMA at fork() time as it is not in use yet and there is no
>   * chance of the global counters getting corrupted as a result of the values.
> 
> 
> Otherwise this patch looks good to me. Thanks.

Will update, Thank you!
-- 
Mike Kravetz