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

Miaohe Lin
2022-09-05T02:32:30Z

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