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 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():
                 /*
                  * 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 */
                         if (!folio_test_large(folio))
                                 mlock_vma_folio(folio, vma);
                         page_vma_mapped_walk_done(&pvmw);
                         ret = false;
                         break;
                 }