Re: [PATCH 7/8] hugetlb: create hugetlb_unmap_file_folio to unmap single file folio

[Miaohe Lin]

On 2022/8/30 6:37, Mike Kravetz wrote:
> On 08/29/22 10:44, Miaohe Lin wrote:
>> On 2022/8/25 1:57, Mike Kravetz wrote:
>>> Create the new routine hugetlb_unmap_file_folio that will unmap a single
>>> file folio.  This is refactored code from hugetlb_vmdelete_list.  It is
>>> modified to do locking within the routine itself and check whether the
>>> page is mapped within a specific vma before unmapping.
>>>
>>> This refactoring will be put to use and expanded upon in a subsequent
>>> patch adding vma specific locking.
>>>
>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>> ---
>>>  fs/hugetlbfs/inode.c | 123 +++++++++++++++++++++++++++++++++----------
>>>  1 file changed, 94 insertions(+), 29 deletions(-)
>>>
>>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>>> index e83fd31671b3..b93d131b0cb5 100644
>>> --- a/fs/hugetlbfs/inode.c
>>> +++ b/fs/hugetlbfs/inode.c
>>> @@ -371,6 +371,94 @@ static void hugetlb_delete_from_page_cache(struct page *page)
>>>  	delete_from_page_cache(page);
>>>  }
>>>  
>>> +/*
>>> + * Called with i_mmap_rwsem held for inode based vma maps.  This makes
>>> + * sure vma (and vm_mm) will not go away.  We also hold the hugetlb fault
>>> + * mutex for the page in the mapping.  So, we can not race with page being
>>> + * faulted into the vma.
>>> + */
>>> +static bool hugetlb_vma_maps_page(struct vm_area_struct *vma,
>>> +				unsigned long addr, struct page *page)
>>> +{
>>> +	pte_t *ptep, pte;
>>> +
>>> +	ptep = huge_pte_offset(vma->vm_mm, addr,
>>> +			huge_page_size(hstate_vma(vma)));
>>> +
>>> +	if (!ptep)
>>> +		return false;
>>> +
>>> +	pte = huge_ptep_get(ptep);
>>> +	if (huge_pte_none(pte) || !pte_present(pte))
>>> +		return false;
>>> +
>>> +	if (pte_page(pte) == page)
>>> +		return true;
>>
>> I'm thinking whether pte entry could change after we check it since huge_pte_lock is not held here.
>> But I think holding i_mmap_rwsem in writelock mode should give us such a guarantee, e.g. migration
>> entry is changed back to huge pte entry while holding i_mmap_rwsem in readlock mode.
>> Or am I miss something?
> 
> Let me think about this.  I do not think it is possible, but you ask good
> questions.
> 
> Do note that this is the same locking sequence used at the beginning of the
> page fault code where the decision to call hugetlb_no_page() is made.

Yes, hugetlb_fault() can tolerate the stale pte entry because pte entry will be re-checked later under the page table lock.
However if we see a stale pte entry here, the page might be leftover after truncated and thus break truncation? But I'm not
sure whether this will occur. Maybe the i_mmap_rwsem writelock and hugetlb_fault_mutex can prevent this issue.

Thanks,
Miaohe Lin


> 
>>
>>> +
>>> +	return false;
>>> +}
>>> +
>>> +/*
>>> + * Can vma_offset_start/vma_offset_end overflow on 32-bit arches?
>>> + * No, because the interval tree returns us only those vmas
>>> + * which overlap the truncated area starting at pgoff,
>>> + * and no vma on a 32-bit arch can span beyond the 4GB.
>>> + */
>>> +static unsigned long vma_offset_start(struct vm_area_struct *vma, pgoff_t start)
>>> +{
>>> +	if (vma->vm_pgoff < start)
>>> +		return (start - vma->vm_pgoff) << PAGE_SHIFT;
>>> +	else
>>> +		return 0;
>>> +}
>>> +
>>> +static unsigned long vma_offset_end(struct vm_area_struct *vma, pgoff_t end)
>>> +{
>>> +	unsigned long t_end;
>>> +
>>> +	if (!end)
>>> +		return vma->vm_end;
>>> +
>>> +	t_end = ((end - vma->vm_pgoff) << PAGE_SHIFT) + vma->vm_start;
>>> +	if (t_end > vma->vm_end)
>>> +		t_end = vma->vm_end;
>>> +	return t_end;
>>> +}
>>> +
>>> +/*
>>> + * Called with hugetlb fault mutex held.  Therefore, no more mappings to
>>> + * this folio can be created while executing the routine.
>>> + */
>>> +static void hugetlb_unmap_file_folio(struct hstate *h,
>>> +					struct address_space *mapping,
>>> +					struct folio *folio, pgoff_t index)
>>> +{
>>> +	struct rb_root_cached *root = &mapping->i_mmap;
>>> +	struct page *page = &folio->page;
>>> +	struct vm_area_struct *vma;
>>> +	unsigned long v_start;
>>> +	unsigned long v_end;
>>> +	pgoff_t start, end;
>>> +
>>> +	start = index * pages_per_huge_page(h);
>>> +	end = ((index + 1) * pages_per_huge_page(h));
>>
>> It seems the outer parentheses is unneeded?
> 
> Correct.  Thanks.
>