On 2021/4/9 6:40, Mike Kravetz wrote: > On 4/7/21 7:44 PM, Miaohe Lin wrote: >> 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... >> > > Please prepare v2 with the changes to remove the HPAGE_RESV_OWNER check > and move the large comment. > Sure. Will do. Thanks. > > I would prefer to leave other places that mention HPAGE_RESV_OWNER > unchanged. > > Thanks, >
