Re: [PATCH 8/8] hugetlb: use new vma_lock for pmd sharing synchronization

Miaohe Lin
2022-09-13T02:15:05Z

On 2022/9/13 7:02, Mike Kravetz wrote:
> On 09/05/22 11:08, Miaohe Lin wrote:
>> On 2022/9/3 7:07, Mike Kravetz wrote:
>>> On 08/30/22 10:02, Miaohe Lin wrote:
>>>> On 2022/8/25 1:57, Mike Kravetz wrote:
>>>>> The new hugetlb vma lock (rw semaphore) is used to address this race:
>>>>>
>>>>> Faulting thread Unsharing thread
>>>>> ... ...
>>>>> ptep = huge_pte_offset()
>>>>> or
>>>>> ptep = huge_pte_alloc()
>>>>> ...
>>>>> i_mmap_lock_write
>>>>> lock page table
>>>>> ptep invalid <------------------------ huge_pmd_unshare()
>>>>> Could be in a previously unlock_page_table
>>>>> sharing process or worse i_mmap_unlock_write
>>>>> ...
>>>>>
>>>>> The vma_lock is used as follows:
>>>>> - During fault processing. the lock is acquired in read mode before
>>>>> doing a page table lock and allocation (huge_pte_alloc). The lock is
>>>>> held until code is finished with the page table entry (ptep).
>>>>> - The lock must be held in write mode whenever huge_pmd_unshare is
>>>>> called.
>>>>>
>>>>> Lock ordering issues come into play when unmapping a page from all
>>>>> vmas mapping the page. The i_mmap_rwsem must be held to search for the
>>>>> vmas, and the vma lock must be held before calling unmap which will
>>>>> call huge_pmd_unshare. This is done today in:
>>>>> - try_to_migrate_one and try_to_unmap_ for page migration and memory
>>>>> error handling. In these routines we 'try' to obtain the vma lock and
>>>>> fail to unmap if unsuccessful. Calling routines already deal with the
>>>>> failure of unmapping.
>>>>> - hugetlb_vmdelete_list for truncation and hole punch. This routine
>>>>> also tries to acquire the vma lock. If it fails, it skips the
>>>>> unmapping. However, we can not have file truncation or hole punch
>>>>> fail because of contention. After hugetlb_vmdelete_list, truncation
>>>>> and hole punch call remove_inode_hugepages. remove_inode_hugepages
>>>>> check for mapped pages and call hugetlb_unmap_file_page to unmap them.
>>>>> hugetlb_unmap_file_page is designed to drop locks and reacquire in the
>>>>> correct order to guarantee unmap success.
>>>>>
>>>>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
>>>>> ---
>>>>> fs/hugetlbfs/inode.c | 46 +++++++++++++++++++
>>>>> mm/hugetlb.c | 102 +++++++++++++++++++++++++++++++++++++++----
>>>>> mm/memory.c | 2 +
>>>>> mm/rmap.c | 100 +++++++++++++++++++++++++++---------------
>>>>> mm/userfaultfd.c | 9 +++-
>>>>> 5 files changed, 214 insertions(+), 45 deletions(-)
>>>>>
>>>>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>>>>> index b93d131b0cb5..52d9b390389b 100644
>>>>> --- a/fs/hugetlbfs/inode.c
>>>>> +++ b/fs/hugetlbfs/inode.c
>>>>> @@ -434,6 +434,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
>>>>> struct folio *folio, pgoff_t index)
>>>>> {
>>>>> struct rb_root_cached *root = &mapping->i_mmap;
>>>>> + unsigned long skipped_vm_start;
>>>>> + struct mm_struct *skipped_mm;
>>>>> struct page *page = &folio->page;
>>>>> struct vm_area_struct *vma;
>>>>> unsigned long v_start;
>>>>> @@ -444,6 +446,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
>>>>> end = ((index + 1) * pages_per_huge_page(h));
>>>>>
>>>>> i_mmap_lock_write(mapping);
>>>>> +retry:
>>>>> + skipped_mm = NULL;
>>>>>
>>>>> vma_interval_tree_foreach(vma, root, start, end - 1) {
>>>>> v_start = vma_offset_start(vma, start);
>>>>> @@ -452,11 +456,49 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
>>>>> if (!hugetlb_vma_maps_page(vma, vma->vm_start + v_start, page))
>>>>> continue;
>>>>>
>>>>> + if (!hugetlb_vma_trylock_write(vma)) {
>>>>> + /*
>>>>> + * If we can not get vma lock, we need to drop
>>>>> + * immap_sema and take locks in order.
>>>>> + */
>>>>> + skipped_vm_start = vma->vm_start;
>>>>> + skipped_mm = vma->vm_mm;
>>>>> + /* grab mm-struct as we will be dropping i_mmap_sema */
>>>>> + mmgrab(skipped_mm);
>>>>> + break;
>>>>> + }
>>>>> +
>>>>> unmap_hugepage_range(vma, vma->vm_start + v_start, v_end,
>>>>> NULL, ZAP_FLAG_DROP_MARKER);
>>>>> + hugetlb_vma_unlock_write(vma);
>>>>> }
>>>>>
>>>>> i_mmap_unlock_write(mapping);
>>>>> +
>>>>> + if (skipped_mm) {
>>>>> + mmap_read_lock(skipped_mm);
>>>>> + vma = find_vma(skipped_mm, skipped_vm_start);
>>>>> + if (!vma || !is_vm_hugetlb_page(vma) ||
>>>>> + vma->vm_file->f_mapping != mapping ||
>>>>> + vma->vm_start != skipped_vm_start) {
>>>>
>>>> i_mmap_lock_write(mapping) is missing here? Retry logic will do i_mmap_unlock_write(mapping) anyway.
>>>>
>>>
>>> Yes, that is missing. I will add here.
>>>
>>>>> + mmap_read_unlock(skipped_mm);
>>>>> + mmdrop(skipped_mm);
>>>>> + goto retry;
>>>>> + }
>>>>> +
>>>>
>>>> IMHO, above check is not enough. Think about the below scene:
>>>>
>>>> CPU 1 CPU 2
>>>> hugetlb_unmap_file_folio exit_mmap
>>>> mmap_read_lock(skipped_mm); mmap_read_lock(mm);
>>>> check vma is wanted.
>>>> unmap_vmas
>>>> mmap_read_unlock(skipped_mm); mmap_read_unlock
>>>> mmap_write_lock(mm);
>>>> free_pgtables
>>>> remove_vma
>>>> hugetlb_vma_lock_free
>>>> vma, hugetlb_vma_lock is still *used after free*
>>>> mmap_write_unlock(mm);
>>>> So we should check mm->mm_users == 0 to fix the above issue. Or am I miss something?
>>>
>>> In the retry case, we are OK because go back and look up the vma again. Right?
>>>
>>> After taking mmap_read_lock, vma can not go away until we mmap_read_unlock.
>>> Before that, we do the following:
>>>
>>>>> + hugetlb_vma_lock_write(vma);
>>>>> + i_mmap_lock_write(mapping);
>>>
>>> IIUC, vma can not go away while we hold i_mmap_lock_write. So, after this we
>>
>> I think you're right. free_pgtables() can't complete its work as unlink_file_vma() will be
>> blocked on i_mmap_rwsem of mapping. Sorry for reporting such nonexistent race.
>>
>>> can.
>>>
>>>>> + mmap_read_unlock(skipped_mm);
>>>>> + mmdrop(skipped_mm);
>>>
>>> We continue to hold i_mmap_lock_write as we goto retry.
>>>
>>> I could be missing something as well. This was how I intended to keep
>>> vma valid while dropping and acquiring locks.
>>
>> Thanks for your clarifying.
>>
>
> Well, that was all correct 'in theory' but not in practice. I did not take
> into account the inode lock that is taken at the beginning of truncate (or
> hole punch). In other code paths, we take inode lock after mmap_lock. So,
> taking mmap_lock here is not allowed.

Considering the Lock ordering in mm/filemap.c:

* ->i_rwsem
* ->invalidate_lock (acquired by fs in truncate path)
* ->i_mmap_rwsem (truncate->unmap_mapping_range)

* ->i_rwsem (generic_perform_write)
* ->mmap_lock (fault_in_readable->do_page_fault)

It seems inode_lock is taken before the mmap_lock?

Thanks,
Miaohe Lin

>
> I came up with another way to make this work. As discussed above, we need to
> drop the i_mmap lock before acquiring the vma_lock. However, once we drop
> i_mmap, the vma could go away. My solution is to make the 'vma_lock' be a
> ref counted structure that can live on after the vma is freed. Therefore,
> this code can take a reference while under i_mmap then drop i_mmap and wait
> on the vma_lock. Of course, once it acquires the vma_lock it needs to check
> and make sure the vma still exists. It may sound complicated, but I think
> it is a bit simpler than the code here. A new series will be out soon.
>