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

[Miaohe Lin <linmiaohe@xxxxxxxxxx>]

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@xxxxxxxxxx>

<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)))

> +	 */
> +	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?

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.

Thanks,
Miaohe Lin