Re: [PATCH v4 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/5/7 19:37, Lance Yang wrote:
> On Tue, May 7, 2024 at 5:33 PM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2024/5/7 16:26, Lance Yang wrote:
>>> On Tue, May 7, 2024 at 2:32 PM Lance Yang <ioworker0@gmail.com> wrote:
>>>>
>>>> Hey Baolin,
>>>>
>>>> Thanks a lot for taking time to review!
>>>>
>>>> On Tue, May 7, 2024 at 12:01 PM Baolin Wang
>>>> <baolin.wang@linux.alibaba.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 2024/5/1 12:27, Lance Yang wrote:
>>>>>> When the user no longer requires the pages, they would use
>>>>>> madvise(MADV_FREE) to mark the pages as lazy free. Subsequently, they
>>>>>> typically would not re-write to that memory again.
>>>>>>
>>>>>> During memory reclaim, if we detect that the large folio and its PMD are
>>>>>> both still marked as clean and there are no unexpected references
>>>>>> (such as GUP), so we can just discard the memory lazily, improving the
>>>>>> efficiency of memory reclamation in this case.  On an Intel i5 CPU, reclaiming 1GiB of lazyfree THPs using
>>>>>> mem_cgroup_force_empty() results in the following runtimes in seconds
>>>>>> (shorter is better):
>>>>>>
>>>>>> --------------------------------------------
>>>>>> |     Old       |      New       |  Change  |
>>>>>> --------------------------------------------
>>>>>> |   0.683426    |    0.049197    |  -92.80% |
>>>>>> --------------------------------------------
>>>>>>
>>>>>> Suggested-by: Zi Yan <ziy@nvidia.com>
>>>>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>>>>> Signed-off-by: Lance Yang <ioworker0@gmail.com>
>>>>>> ---
>>>>>>     include/linux/huge_mm.h |  9 +++++
>>>>>>     mm/huge_memory.c        | 73 +++++++++++++++++++++++++++++++++++++++++
>>>>>>     mm/rmap.c               |  3 ++
>>>>>>     3 files changed, 85 insertions(+)
>>>>>>
>>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>>> index 38c4b5537715..017cee864080 100644
>>>>>> --- a/include/linux/huge_mm.h
>>>>>> +++ b/include/linux/huge_mm.h
>>>>>> @@ -411,6 +411,8 @@ static inline bool thp_migration_supported(void)
>>>>>>
>>>>>>     void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
>>>>>>                            pmd_t *pmd, bool freeze, struct folio *folio);
>>>>>> +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
>>>>>> +                        pmd_t *pmdp, struct folio *folio);
>>>>>>
>>>>>>     static inline void align_huge_pmd_range(struct vm_area_struct *vma,
>>>>>>                                         unsigned long *start,
>>>>>> @@ -492,6 +494,13 @@ static inline void align_huge_pmd_range(struct vm_area_struct *vma,
>>>>>>                                         unsigned long *start,
>>>>>>                                         unsigned long *end) {}
>>>>>>
>>>>>> +static inline bool unmap_huge_pmd_locked(struct vm_area_struct *vma,
>>>>>> +                                      unsigned long addr, pmd_t *pmdp,
>>>>>> +                                      struct folio *folio)
>>>>>> +{
>>>>>> +     return false;
>>>>>> +}
>>>>>> +
>>>>>>     #define split_huge_pud(__vma, __pmd, __address)     \
>>>>>>         do { } while (0)
>>>>>>
>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>> index 145505a1dd05..90fdef847a88 100644
>>>>>> --- a/mm/huge_memory.c
>>>>>> +++ b/mm/huge_memory.c
>>>>>> @@ -2690,6 +2690,79 @@ static void unmap_folio(struct folio *folio)
>>>>>>         try_to_unmap_flush();
>>>>>>     }
>>>>>>
>>>>>> +static bool __discard_trans_pmd_locked(struct vm_area_struct *vma,
>>>>>> +                                    unsigned long addr, pmd_t *pmdp,
>>>>>> +                                    struct folio *folio)
>>>>>> +{
>>>>>> +     struct mm_struct *mm = vma->vm_mm;
>>>>>> +     int ref_count, map_count;
>>>>>> +     pmd_t orig_pmd = *pmdp;
>>>>>> +     struct mmu_gather tlb;
>>>>>> +     struct page *page;
>>>>>> +
>>>>>> +     if (pmd_dirty(orig_pmd) || folio_test_dirty(folio))
>>>>>> +             return false;
>>>>>> +     if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd)))
>>>>>> +             return false;
>>>>>> +
>>>>>> +     page = pmd_page(orig_pmd);
>>>>>> +     if (unlikely(page_folio(page) != folio))
>>>>>> +             return false;
>>>>>> +
>>>>>> +     tlb_gather_mmu(&tlb, mm);
>>>>>> +     orig_pmd = pmdp_huge_get_and_clear(mm, addr, pmdp);
>>>>>> +     tlb_remove_pmd_tlb_entry(&tlb, pmdp, addr);
>>>>>> +
>>>>>> +     /*
>>>>>> +      * Syncing against concurrent GUP-fast:
>>>>>> +      * - clear PMD; barrier; read refcount
>>>>>> +      * - inc refcount; barrier; read PMD
>>>>>> +      */
>>>>>> +     smp_mb();
>>>>>> +
>>>>>> +     ref_count = folio_ref_count(folio);
>>>>>> +     map_count = folio_mapcount(folio);
>>>>>> +
>>>>>> +     /*
>>>>>> +      * Order reads for folio refcount and dirty flag
>>>>>> +      * (see comments in __remove_mapping()).
>>>>>> +      */
>>>>>> +     smp_rmb();
>>>>>> +
>>>>>> +     /*
>>>>>> +      * If the PMD or folio is redirtied at this point, or if there are
>>>>>> +      * unexpected references, we will give up to discard this folio
>>>>>> +      * and remap it.
>>>>>> +      *
>>>>>> +      * The only folio refs must be one from isolation plus the rmap(s).
>>>>>> +      */
>>>>>> +     if (ref_count != map_count + 1 || folio_test_dirty(folio) ||
>>>>>> +         pmd_dirty(orig_pmd)) {
>>>>>> +             set_pmd_at(mm, addr, pmdp, orig_pmd);
>>>>>> +             return false;
>>>>>> +     }
>>>>>> +
>>>>>> +     folio_remove_rmap_pmd(folio, page, vma);
>>>>>> +     zap_deposited_table(mm, pmdp);
>>>>>> +     add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
>>>>>> +     folio_put(folio);
>>>>>
>>>>> IIUC, you missed handling mlock vma, see mlock_drain_local() in
>>>>> try_to_unmap_one().
>>>>
>>>> Good spot!
>>>>
>>>> I suddenly realized that I overlooked another thing: If we detect that a
>>>> PMD-mapped THP is within the range of the VM_LOCKED VMA, we
>>>> should check whether the TTU_IGNORE_MLOCK flag is set in
>>>> try_to_unmap_one(). If the flag is set, we will remove the PMD mapping
>>>> from the folio. Otherwise, the folio should be mlocked, which avoids
>>>> splitting the folio and then mlocking each page again.
>>>
>>> My previous response above is flawed - sorry :(
>>>
>>> If we detect that a PMD-mapped THP is within the range of the
>>> VM_LOCKED VMA.
>>>
>>> 1) If the TTU_IGNORE_MLOCK flag is set, we will try to remove the
>>> PMD mapping from the folio, as this series has done.
>>
>> Right.
>>
>>> 2) If the flag is not set, the large folio should be mlocked to prevent it
>>> from being picked during memory reclaim? Currently, we just leave it
>>
>> Yes. From commit 1acbc3f93614 ("mm: handle large folio when large folio
>> in VM_LOCKED VMA range"), large folios of the mlocked VMA will be
>> handled during page reclaim phase.
>>
>>> as is and do not to mlock it, IIUC.
>>
>> Original code already handle the mlock case after the PMD-mapped THP is
>> split in try_to_unmap_one():
> 
> Yep. But this series doesn't do the TTU_SPLIT_HUGE_PMD immediately.
> 
>>                   /*
>>                    * If the folio is in an mlock()d vma, we must not swap
>> it out.
>>                    */
>>                   if (!(flags & TTU_IGNORE_MLOCK) &&
>>                       (vma->vm_flags & VM_LOCKED)) {
>>                           /* Restore the mlock which got missed */
> 
> IIUC, we could detect a PMD-mapped THP here. So, I'm not sure if we
> need to mlock it to prevent it from being picked again during memory
> reclaim. The change is as follows:

For the page reclaim path, folio_check_references() should be able to 
help restore the mlock of the PMD-mapped THP. However, for other paths 
that call try_to_unmap(), I believe it is still necessary to check 
whether the mlock of the PMD-mapped THP was missed.

Below code looks reasonable to me from a quick glance.

> diff --git a/mm/rmap.c b/mm/rmap.c
> index ed7f82036986..2a9d037ab23c 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1673,7 +1673,8 @@ static bool try_to_unmap_one(struct folio
> *folio, struct vm_area_struct *vma,
>                  if (!(flags & TTU_IGNORE_MLOCK) &&
>                      (vma->vm_flags & VM_LOCKED)) {
>                          /* Restore the mlock which got missed */
> -                       if (!folio_test_large(folio))
> +                       if (!folio_test_large(folio) ||
> +                           (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
>                                  mlock_vma_folio(folio, vma);
>                          goto walk_done_err;
>                  }
>