Messages in this thread | | | Subject | Re: [PATCH] mm: fix account pmd page to the process | From | Mike Kravetz <> | Date | Thu, 16 Jun 2016 09:05:23 -0700 |
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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@huawei.com> >>> >>> 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@huawei.com> >>> --- >>> 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).
I think that entire if/else statement can be removed. We know pud_none(*pud), so just do pud_populate().
-- Mike Kravetz
>> >>> + >>> spin_unlock(ptl); >>> out: >>> pte = (pte_t *)pmd_alloc(mm, pud, addr); >>> -- >>> 1.8.3.1 >> >> -- >> Michal Hocko >> SUSE Labs >
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