Messages in this thread | | | Date | Thu, 9 May 2024 17:36:01 +0800 | Subject | Re: [PATCH v4 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list() | From | Baolin Wang <> |
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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; > } >
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