On 20 Jul 2021, at 16:53, Yang Shi wrote:
> On Tue, Jul 20, 2021 at 7:25 AM Christian Borntraeger
> <borntraeger@xxxxxxxxxx> wrote:
>>
>>
>>
>> On 20.07.21 08:55, Huang Ying wrote:
>>> Before the commit c5b5a3dd2c1f ("mm: thp: refactor NUMA fault
>>> handling"), the TLB flushing is done in do_huge_pmd_numa_page() itself
>>> via flush_tlb_range().
>>>
>>> But after commit c5b5a3dd2c1f ("mm: thp: refactor NUMA fault
>>> handling"), the TLB flushing is done in migrate_pages() as in the
>>> following code path anyway.
>>>
>>> do_huge_pmd_numa_page
>>> migrate_misplaced_page
>>> migrate_pages
>>>
>>> So now, the TLB flushing code in do_huge_pmd_numa_page() becomes
>>> unnecessary. So the code is deleted in this patch to simplify the
>>> code. This is only code cleanup, there's no visible performance
>>> difference.
>>>
>>> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>>> Cc: Yang Shi <shy828301@xxxxxxxxx>
>>> Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
>>> Cc: Mel Gorman <mgorman@xxxxxxx>
>>> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>>> Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
>>> Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
>>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>>> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
>>> Cc: Zi Yan <ziy@xxxxxxxxxx>
>>> ---
>>> mm/huge_memory.c | 26 --------------------------
>>> 1 file changed, 26 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index afff3ac87067..9f21e44c9030 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -1440,32 +1440,6 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>> goto out;
>>> }
>>>
>>> - /*
>>> - * Since we took the NUMA fault, we must have observed the !accessible
>>> - * bit. Make sure all other CPUs agree with that, to avoid them
>>> - * modifying the page we're about to migrate.
>>> - *
>>> - * Must be done under PTL such that we'll observe the relevant
>>> - * inc_tlb_flush_pending().
>>> - *
>>> - * We are not sure a pending tlb flush here is for a huge page
>>> - * mapping or not. Hence use the tlb range variant
>>> - */
>>> - if (mm_tlb_flush_pending(vma->vm_mm)) {
>>> - flush_tlb_range(vma, haddr, haddr + HPAGE_PMD_SIZE);
>>> - /*
>>> - * change_huge_pmd() released the pmd lock before
>>> - * invalidating the secondary MMUs sharing the primary
>>> - * MMU pagetables (with ->invalidate_range()). The
>>> - * mmu_notifier_invalidate_range_end() (which
>>> - * internally calls ->invalidate_range()) in
>>> - * change_pmd_range() will run after us, so we can't
>>> - * rely on it here and we need an explicit invalidate.
>>> - */
>>> - mmu_notifier_invalidate_range(vma->vm_mm, haddr,
>>> - haddr + HPAGE_PMD_SIZE);
>>> - }
>>> CC Paolo/KVM list so we also remove the mmu notifier here. Do we need those
>> now in migrate_pages? I am not an expert in that code, but I cant find
>> an equivalent mmu_notifier in migrate_misplaced_pages.
>> I might be totally wrong, just something that I noticed.
>
> Do you mean the missed mmu notifier invalidate for the THP migration
> case? Yes, I noticed that too. But I'm not sure whether it is intended
> or just missed.
From my understand of mmu_notifier document, mmu_notifier_invalidate_range()
is needed only if the PTE is updated to point to a new page or the page pointed
by the PTE is freed. Page migration does not fall into either case.
In addition, in migrate_pages(), more specifically try_to_migrate_one(),
there is a pair of mmu_notifier_invalidate_range_start() and
mmu_notifier_invalidate_range_end() around the PTE manipulation code, which should
be sufficient to notify secondary TLBs (including KVM) about the PTE change
for page migration. Correct me if I am wrong.
—
Best Regards,
Yan, Zi
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