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. I would prefer to leave other places that mention HPAGE_RESV_OWNER unchanged. Thanks, -- Mike Kravetz
