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

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/9/3 5:35, Mike Kravetz wrote:
> On 08/30/22 10:46, Miaohe Lin wrote:
>> 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@oracle.com>
>>>>> ---
>>>>>  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.
>>
> 
> I looked at this some more.  Just to be clear, we only need to worry
> about modifications of pte_page().  Racing with other pte modifications
> such as accessed, or protection changes is acceptable.
> 
> Of course, the fault mutex prevents faults from happening.  i_mmap_rwsem
> protects against unmap and truncation operations as well as migration as
> you noted above.  I believe the only other place where we update pte_page()
> is when copying page table such as during fork.  However, with commit
> bcd51a3c679d "Lazy page table copies in fork()" we are going to skip
> copying for files and rely on page faults to populate the tables.
> 
> I believe we are safe from races with just the fault mutex and i_mmap_rwsem.

I believe your analysis is right. Thanks for your clarifying.

Thanks,
Miaohe Lin