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

[Mike Kravetz]

On 09/13/22 10:14, Miaohe Lin wrote:
> 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@xxxxxxxxxx>
> >>>>> ---
> >>>>>  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?

Hmmmm?  I can't find a sequence where inode_lock is taken after mmap_lock.
lockdep was complaining about taking mmap_lock after i_rwsem in the above code.
I assumed there was such a sequence somewhere.  Might need to go back and get
another trace/warning.

In any case, I think the scheme below is much cleaner.  Doing another round of
benchmarking before sending.

> > 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.
> > 

-- 
Mike Kravetz