Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte_range to handle large folio

[Yin Fengwei]

On 7/27/23 00:57, Yu Zhao wrote:
> On Wed, Jul 26, 2023 at 6:49 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>
>>
>>
>> On 7/15/23 14:06, Yu Zhao wrote:
>>> On Wed, Jul 12, 2023 at 12:31 AM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>>>>
>>>> On Wed, Jul 12, 2023 at 12:02 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>>
>>>>> Current kernel only lock base size folio during mlock syscall.
>>>>> Add large folio support with following rules:
>>>>>   - Only mlock large folio when it's in VM_LOCKED VMA range
>>>>>
>>>>>   - If there is cow folio, mlock the cow folio as cow folio
>>>>>     is also in VM_LOCKED VMA range.
>>>>>
>>>>>   - munlock will apply to the large folio which is in VMA range
>>>>>     or cross the VMA boundary.
>>>>>
>>>>> The last rule is used to handle the case that the large folio is
>>>>> mlocked, later the VMA is split in the middle of large folio
>>>>> and this large folio become cross VMA boundary.
>>>>>
>>>>> Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
>>>>> ---
>>>>>  mm/mlock.c | 104 ++++++++++++++++++++++++++++++++++++++++++++++++++---
>>>>>  1 file changed, 99 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/mm/mlock.c b/mm/mlock.c
>>>>> index 0a0c996c5c214..f49e079066870 100644
>>>>> --- a/mm/mlock.c
>>>>> +++ b/mm/mlock.c
>>>>> @@ -305,6 +305,95 @@ void munlock_folio(struct folio *folio)
>>>>>         local_unlock(&mlock_fbatch.lock);
>>>>>  }
>>>>>
>>>>> +static inline bool should_mlock_folio(struct folio *folio,
>>>>> +                                       struct vm_area_struct *vma)
>>>>> +{
>>>>> +       if (vma->vm_flags & VM_LOCKED)
>>>>> +               return (!folio_test_large(folio) ||
>>>>> +                               folio_within_vma(folio, vma));
>>>>> +
>>>>> +       /*
>>>>> +        * For unlock, allow munlock large folio which is partially
>>>>> +        * mapped to VMA. As it's possible that large folio is
>>>>> +        * mlocked and VMA is split later.
>>>>> +        *
>>>>> +        * During memory pressure, such kind of large folio can
>>>>> +        * be split. And the pages are not in VM_LOCKed VMA
>>>>> +        * can be reclaimed.
>>>>> +        */
>>>>> +
>>>>> +       return true;
>>>>
>>>> Looks good, or just
>>>>
>>>> should_mlock_folio() // or whatever name you see fit, can_mlock_folio()?
>>>> {
>>>>   return !(vma->vm_flags & VM_LOCKED) || folio_within_vma();
>>>> }
>>>>
>>>>> +}
>>>>> +
>>>>> +static inline unsigned int get_folio_mlock_step(struct folio *folio,
>>>>> +                       pte_t pte, unsigned long addr, unsigned long end)
>>>>> +{
>>>>> +       unsigned int nr;
>>>>> +
>>>>> +       nr = folio_pfn(folio) + folio_nr_pages(folio) - pte_pfn(pte);
>>>>> +       return min_t(unsigned int, nr, (end - addr) >> PAGE_SHIFT);
>>>>> +}
>>>>> +
>>>>> +void mlock_folio_range(struct folio *folio, struct vm_area_struct *vma,
>>>>> +               pte_t *pte, unsigned long addr, unsigned int nr)
>>>>> +{
>>>>> +       struct folio *cow_folio;
>>>>> +       unsigned int step = 1;
>>>>> +
>>>>> +       mlock_folio(folio);
>>>>> +       if (nr == 1)
>>>>> +               return;
>>>>> +
>>>>> +       for (; nr > 0; pte += step, addr += (step << PAGE_SHIFT), nr -= step) {
>>>>> +               pte_t ptent;
>>>>> +
>>>>> +               step = 1;
>>>>> +               ptent = ptep_get(pte);
>>>>> +
>>>>> +               if (!pte_present(ptent))
>>>>> +                       continue;
>>>>> +
>>>>> +               cow_folio = vm_normal_folio(vma, addr, ptent);
>>>>> +               if (!cow_folio || cow_folio == folio) {
>>>>> +                       continue;
>>>>> +               }
>>>>> +
>>>>> +               mlock_folio(cow_folio);
>>>>> +               step = get_folio_mlock_step(folio, ptent,
>>>>> +                               addr, addr + (nr << PAGE_SHIFT));
>>>>> +       }
>>>>> +}
>>>>> +
>>>>> +void munlock_folio_range(struct folio *folio, struct vm_area_struct *vma,
>>>>> +               pte_t *pte, unsigned long addr, unsigned int nr)
>>>>> +{
>>>>> +       struct folio *cow_folio;
>>>>> +       unsigned int step = 1;
>>>>> +
>>>>> +       munlock_folio(folio);
>>>>> +       if (nr == 1)
>>>>> +               return;
>>>>> +
>>>>> +       for (; nr > 0; pte += step, addr += (step << PAGE_SHIFT), nr -= step) {
>>>>> +               pte_t ptent;
>>>>> +
>>>>> +               step = 1;
>>>>> +               ptent = ptep_get(pte);
>>>>> +
>>>>> +               if (!pte_present(ptent))
>>>>> +                       continue;
>>>>> +
>>>>> +               cow_folio = vm_normal_folio(vma, addr, ptent);
>>>>> +               if (!cow_folio || cow_folio == folio) {
>>>>> +                       continue;
>>>>> +               }
>>>>> +
>>>>> +               munlock_folio(cow_folio);
>>>>> +               step = get_folio_mlock_step(folio, ptent,
>>>>> +                               addr, addr + (nr << PAGE_SHIFT));
>>>>> +       }
>>>>> +}
>>>>
>>>> I'll finish the above later.
>>>
>>> There is a problem here that I didn't have the time to elaborate: we
>>> can't mlock() a folio that is within the range but not fully mapped
>>> because this folio can be on the deferred split queue. When the split
>>> happens, those unmapped folios (not mapped by this vma but are mapped
>>> into other vmas) will be stranded on the unevictable lru.
>> Checked remap case in past few days, I agree we shouldn't treat a folio
>> in the range but not fully mapped as in_range folio.
>>
>> As for remap case, it's possible that the folio is not in deferred split
>> queue. But part of folio is mapped to VM_LOCKED vma and other part of
>> folio is mapped to none VM_LOCKED vma. In this case, page can't be split
>> as it's not in deferred split queue. So page reclaim should be allowed to
>> pick this folio up, split it and reclaim the pages in none VM_LOCKED vma.
>> So we can't mlock such kind of folio.
>>
>> The same thing can happen with madvise_cold_or_pageout_pte_range().
>> I will update folio_in_vma() to check the PTE also.
> 
> Thanks, and I think we should move forward with this series and fix
> the potential mlock race problem separately since it's not caused by
> this series.
> 
> WDYT?

Yes. Agree. Will send v3 with remap case covered.


Regards
Yin, Fengwei