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?
> never want to indicate reservations are available. The ternary makes
> sure a positive value is never returned.
>
Many thanks for review and reply! :)
