On 4/6/21 7:05 PM, Miaohe Lin wrote:
> Hi:
> On 2021/4/7 8:53, Mike Kravetz wrote:
>> On 4/2/21 2:32 AM, Miaohe Lin wrote:
>>> It's guaranteed that the vma is associated with a resv_map, i.e. either
>>> VM_MAYSHARE or HPAGE_RESV_OWNER, when the code reaches here or we would
>>> have returned via !resv check above. So ret must be less than 0 in the
>>> 'else' case. Simplify the return code to make this clear.
>>
>> I believe we still neeed that ternary operator in the return statement.
>> Why?
>>
>> There are two basic types of mappings to be concerned with:
>> shared and private.
>> For private mappings, a task can 'own' the mapping as indicated by
>> HPAGE_RESV_OWNER. Or, it may not own the mapping. The most common way
>> to create a non-owner private mapping is to have a task with a private
>> mapping fork. The parent process will have HPAGE_RESV_OWNER set, the
>> child process will not. The idea is that since the child has a COW copy
>> of the mapping it should not consume reservations made by the parent.
>
> The child process will not have HPAGE_RESV_OWNER set because at fork time, we
> do:
> /*
> * Clear hugetlb-related page reserves for children. This only
> * affects MAP_PRIVATE mappings. Faults generated by the child
> * are not guaranteed to succeed, even if read-only
> */
> if (is_vm_hugetlb_page(tmp))
> reset_vma_resv_huge_pages(tmp);
> i.e. we have vma->vm_private_data = (void *)0; for child process and
> vma_resv_map() will
> return NULL in this case.
> Or am I missed something?
>
>> Only the parent (HPAGE_RESV_OWNER) is allowed to consume the
>> reservations.
>> Hope that makens sense?
>>
>>>
>>> Signed-off-by: Miaohe Lin <linmia...@huawei.com>
>>> ---
>>> mm/hugetlb.c | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index a03a50b7c410..b7864abded3d 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -2183,7 +2183,7 @@ static long __vma_reservation_common(struct hstate *h,
>>> return 1;
>>> }
>>> else
>>
>> This else also handles the case !HPAGE_RESV_OWNER. In this case, we
>
> IMO, for the case !HPAGE_RESV_OWNER, we won't reach here. What do you think?
>
I think you are correct.
However, if this is true we should be able to simply the code even
further. There is no need to check for HPAGE_RESV_OWNER because we know
it must be set. Correct? If so, the code could look something like:
if (vma->vm_flags & VM_MAYSHARE)
return ret;
/* We know private mapping with HPAGE_RESV_OWNER */
* ... *
* Add that existing comment */
if (ret > 0)
return 0;
if (ret == 0)
return 1;
return ret;
--
Mike Kravetz
