On 2021/4/8 5:23, Mike Kravetz wrote: > On 4/6/21 8:09 PM, Miaohe Lin wrote: >> On 2021/4/7 10:37, Mike Kravetz wrote: >>> 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 <linmiaohe@xxxxxxxxxx> >>>>>> --- >>>>>> 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; >>> >> >> Many thanks for good suggestion! What do you mean is this ? > > I think the below changes would work fine. > > However, this patch/discussion has made me ask the question. Do we need > the HPAGE_RESV_OWNER flag? Is the followng true? > !(vm_flags & VM_MAYSHARE) && vma_resv_map() ===> HPAGE_RESV_OWNER > !(vm_flags & VM_MAYSHARE) && !vma_resv_map() ===> !HPAGE_RESV_OWNER > I agree with you. HPAGE_RESV_OWNER is set in hugetlb_reserve_pages() and there's no way to clear it in the owner process. The child process can not inherit both HPAGE_RESV_OWNER and resv_map. So for !HPAGE_RESV_OWNER vma, it knows nothing about resv_map. IMO, in !(vm_flags & VM_MAYSHARE) case, we must have: !!vma_resv_map() == !!HPAGE_RESV_OWNER > I am not suggesting we eliminate the flag and make corresponding > changes. Just curious if you believe we 'could' remove the flag and > depend on the above conditions. > > One reason for NOT removing the flag is that that flag itself and > supporting code and commnets help explain what happens with hugetlb > reserves for COW mappings. That code is hard to understand and the > existing code and coments around HPAGE_RESV_OWNER help with > understanding. Agree. These codes took me several days to understand... > Thanks.
