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

Miaohe Lin
2022-09-05T03:09:51Z

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.

>
>>> +
>>> + v_start = vma_offset_start(vma, start);
>>> + v_end = vma_offset_end(vma, end);
>>> + unmap_hugepage_range(vma, vma->vm_start + v_start, v_end,
>>> + NULL, ZAP_FLAG_DROP_MARKER);
>>> + hugetlb_vma_unlock_write(vma);
>>> +
>>> + goto retry;
>>
>> Should here be one cond_resched() here in case this function will take a really long time?
>>
>
> I think we will at most retry once.

I see. It should be acceptable.

>
>>> + }
>>> }
>>>
>>> static void
>>> @@ -474,11 +516,15 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
>>> unsigned long v_start;
>>> unsigned long v_end;
>>>
>>> + if (!hugetlb_vma_trylock_write(vma))
>>> + continue;
>>> +
>>> v_start = vma_offset_start(vma, start);
>>> v_end = vma_offset_end(vma, end);
>>>
>>> unmap_hugepage_range(vma, vma->vm_start + v_start, v_end,
>>> NULL, zap_flags);
>>> + hugetlb_vma_unlock_write(vma);
>>> }
>>
>> unmap_hugepage_range is not called under hugetlb_vma_lock in unmap_ref_private since it's private vma?
>> Add a comment to avoid future confusion?
>>
>>> }
>
> Sure, will add a comment before hugetlb_vma_lock.
>
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 6fb0bff2c7ee..5912c2b97ddf 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -4801,6 +4801,14 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>>> mmu_notifier_invalidate_range_start(&range);
>>> mmap_assert_write_locked(src);
>>> raw_write_seqcount_begin(&src->write_protect_seq);
>>> + } else {
>>> + /*
>>> + * For shared mappings the vma lock must be held before
>>> + * calling huge_pte_offset in the src vma. Otherwise, the
>>
>> s/huge_pte_offset/huge_pte_alloc/, i.e. huge_pte_alloc could return shared pmd, not huge_pte_offset which
>> might lead to confusion. But this is really trivial...
>
> Actually, it is huge_pte_offset. While looking up ptes in the source vma, we
> do not want to race with other threads in the source process which could
> be doing a huge_pmd_unshare. Otherwise, the returned pte could be invalid.
>
> FYI - Most of this code is now 'dead' because of bcd51a3c679d "Lazy page table
> copies in fork()". We will not copy shared mappigns at fork time.

Agree. Should these "dead" codes be removed later?

Thanks,
Miaohe Lin

>
>>
>> Except from above comments, this patch looks good to me.
>>
>
> Thank you! Thank you! Thank you! For looking at this series and all
> your comments. I hope to send out v2 next week.
>