On 06/16/2016 09:31 AM, Michal Hocko wrote: > On Thu 16-06-16 09:05:23, Mike Kravetz wrote: >> On 06/16/2016 08:43 AM, Michal Hocko wrote: >>> [It seems that this patch has been sent several times and this >>> particular copy didn't add Kirill who has added this code CC him now] >>> >>> On Thu 16-06-16 17:42:14, Michal Hocko wrote: >>>> On Thu 16-06-16 19:36:11, zhongjiang wrote: >>>>> From: zhong jiang <zhongjiang@xxxxxxxxxx> >>>>> >>>>> when a process acquire a pmd table shared by other process, we >>>>> increase the account to current process. otherwise, a race result >>>>> in other tasks have set the pud entry. so it no need to increase it. >>>>> >>>>> Signed-off-by: zhong jiang <zhongjiang@xxxxxxxxxx> >>>>> --- >>>>> mm/hugetlb.c | 5 ++--- >>>>> 1 file changed, 2 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>>> index 19d0d08..3b025c5 100644 >>>>> --- a/mm/hugetlb.c >>>>> +++ b/mm/hugetlb.c >>>>> @@ -4189,10 +4189,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud) >>>>> if (pud_none(*pud)) { >>>>> pud_populate(mm, pud, >>>>> (pmd_t *)((unsigned long)spte & PAGE_MASK)); >>>>> - } else { >>>>> + } else >>>>> put_page(virt_to_page(spte)); >>>>> - mm_inc_nr_pmds(mm); >>>>> - } >>>> >>>> The code is quite puzzling but is this correct? Shouldn't we rather do >>>> mm_dec_nr_pmds(mm) in that path to undo the previous inc? >> >> I agree that the code is quite puzzling. :( >> >> However, if this were an issue I would have expected to see some reports. >> Oracle DB makes use of this feature (shared page tables) and if the pmd >> count is wrong we would catch it in check_mm() at exit time. >> >> Upon closer examination, I believe the code in question is never executed. >> Note the callers of huge_pmd_share. The calling code looks like: >> >> if (want_pmd_share() && pud_none(*pud)) >> pte = huge_pmd_share(mm, addr, pud); >> else >> pte = (pte_t *)pmd_alloc(mm, pud, addr); >> >> Therefore, we do not call huge_pmd_share unless pud_none(*pud). The >> code in question is only executed when !pud_none(*pud). > > My understanding is that the check is needed after we retake page lock > because we might have raced with other thread. But it's been quite some > time since I've looked at hugetlb locking and page table sharing code. That is correct, we could have raced. Duh! In the case of a race, the other thread would have incremented the PMD count already. Your suggestion of decrementing pmd count in this case seems to be the correct approach. But, I need to think about this some more. -- Mike Kravetz -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>