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

Miaohe Lin
2022-07-30T02:12:19Z

On 2022/7/30 2:00, Mike Kravetz wrote:
> 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.

Many thanks for your explanation. :)

>
>>> + 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)))

Does above check need a comment? VM_SHARED is check here but not in below hugetlb_alloc_vma_lock.

Thanks.

>>> + 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!
>