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

[Mike Kravetz]

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.

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