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

Mike Kravetz
2022-07-29T20:15:38Z

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@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?
>

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