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

[Mike Kravetz]

On 07/29/22 10:55, Miaohe Lin wrote:
> 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@xxxxxxxxxx>
> > ---
> >  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?
> 

I 'think' we want them to be different.  Note this subtle code and
explanation in __unmap_hugepage_range_final().

	/*
	 * Clear this flag so that x86's huge_pmd_share page_table_shareable
	 * test will fail on a vma being torn down, and not grab a page table
	 * on its way out.  We're lucky that the flag has such an appropriate
	 * name, and can in fact be safely cleared here. We could clear it
	 * before the __unmap_hugepage_range above, but all that's necessary
	 * is to clear it before releasing the i_mmap_rwsem. This works
	 * because in the context this is called, the VMA is about to be
	 * destroyed and the i_mmap_rwsem is held.
	 */
	vma->vm_flags &= ~VM_MAYSHARE;

So, when making a decision to share or not we need to only check VM_MAYSHARE.
When making decisions about about the vma_lock, we need to check both.  In most
cases, just VM_MAYSHARE would be sufficient but we need to handle this case
where VM_SHARED and !VM_MAYSHARE.  Mostly in the unmap/cleanup cases.

> > +		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)))

Based on my explanation above, I think this should only check VM_MAYSHARE.

> > +		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 ?

Yes, I believe your analysis is correct.

>                                                                                    But
> when hugetlb_alloc_vma_lock is called from hugetlb_reserve_pages, it should work fine.
> Or am I miss something?

You are right.  This is an issue in the current code.  I will address in
the next version.

Thanks for all your comments on this series!
-- 
Mike Kravetz