Re: [RFC PATCH v4 6/8] hugetlb: add vma based lock for pmd sharing synchronization

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/7/7 4:23, 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>
> ---
>  include/linux/hugetlb.h |  36 +++++++++-
>  kernel/fork.c           |   6 +-
>  mm/hugetlb.c            | 150 ++++++++++++++++++++++++++++++++++++----
>  mm/rmap.c               |   8 ++-
>  4 files changed, 178 insertions(+), 22 deletions(-)
> 

<snip>

>  
>  /* Forward declaration */
>  static int hugetlb_acct_memory(struct hstate *h, long delta);
> +static bool vma_pmd_shareable(struct vm_area_struct *vma);
>  
>  static inline bool subpool_is_free(struct hugepage_subpool *spool)
>  {
> @@ -904,6 +905,89 @@ resv_map_set_hugetlb_cgroup_uncharge_info(struct resv_map *resv_map,
>  #endif
>  }
>  
> +static bool __vma_shareable_flags_pmd(struct vm_area_struct *vma)
> +{
> +	return vma->vm_flags & (VM_MAYSHARE | VM_SHARED) &&

Should me make __vma_aligned_range_pmd_shareable check (VM_MAYSHARE | VM_SHARED) like above
instead of VM_MAYSHARE to make code more consistent?

> +		vma->vm_private_data;
> +}
> +
> +void hugetlb_vma_lock_read(struct vm_area_struct *vma)
> +{
> +	if (__vma_shareable_flags_pmd(vma))
> +		down_read((struct rw_semaphore *)vma->vm_private_data);
> +}
> +
> +void hugetlb_vma_unlock_read(struct vm_area_struct *vma)
> +{
> +	if (__vma_shareable_flags_pmd(vma))
> +		up_read((struct rw_semaphore *)vma->vm_private_data);
> +}
> +
> +void hugetlb_vma_lock_write(struct vm_area_struct *vma)
> +{
> +	if (__vma_shareable_flags_pmd(vma))
> +		down_write((struct rw_semaphore *)vma->vm_private_data);
> +}
> +
> +void hugetlb_vma_unlock_write(struct vm_area_struct *vma)
> +{
> +	if (__vma_shareable_flags_pmd(vma))
> +		up_write((struct rw_semaphore *)vma->vm_private_data);
> +}
> +
> +int hugetlb_vma_trylock_write(struct vm_area_struct *vma)
> +{
> +	if (!__vma_shareable_flags_pmd(vma))
> +		return 1;
> +
> +	return down_write_trylock((struct rw_semaphore *)vma->vm_private_data);
> +}
> +
> +void hugetlb_vma_assert_locked(struct vm_area_struct *vma)
> +{
> +	if (__vma_shareable_flags_pmd(vma))
> +		lockdep_assert_held((struct rw_semaphore *)
> +				vma->vm_private_data);
> +}
> +
> +static void hugetlb_free_vma_lock(struct vm_area_struct *vma)
> +{
> +	/* Only present in sharable vmas */
> +	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_alloc_vma_lock(struct vm_area_struct *vma)
> +{
> +	struct rw_semaphore *vma_sema;
> +
> +	/* Only establish in (flags) sharable vmas */
> +	if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
> +		return;
> +> +	if (!vma_pmd_shareable(vma)) {
> +		vma->vm_private_data = NULL;
> +		return;
> +	}
> +
> +	vma_sema = kmalloc(sizeof(*vma_sema), GFP_KERNEL);
> +	if (!vma_sema) {
> +		/*
> +		 * If we can not allocate semaphore, then vma can not
> +		 * participate in pmd sharing.
> +		 */
> +		vma->vm_private_data = NULL;
> +	} else {
> +		init_rwsem(vma_sema);
> +		vma->vm_private_data = vma_sema;
> +	}

This code is really subtle. If it's called from hugetlb_vm_op_open during fork after
hugetlb_dup_vma_private is done, there should already be a kmalloc-ed vma_sema for this
vma (because hugetlb_alloc_vma_lock is also called by hugetlb_dup_vma_private). So we
can't simply change the value of vm_private_data here or vma_sema will be leaked ? But
when hugetlb_alloc_vma_lock is called from hugetlb_reserve_pages, it should work fine.
Or am I miss something?

Thanks.