Re: [PATCH v4 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()
From: Lance Yang
Date: Tue May 07 2024 - 07:38:00 EST
On Tue, May 7, 2024 at 5:33 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2024/5/7 16:26, Lance Yang wrote:
> > On Tue, May 7, 2024 at 2:32 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> >>
> >> Hey Baolin,
> >>
> >> Thanks a lot for taking time to review!
> >>
> >> On Tue, May 7, 2024 at 12:01 PM Baolin Wang
> >> <baolin.wang@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> >>>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> >>>> Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> >>>> ---
> >>>> 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:
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;
}
Thanks,
Lance
> if (!folio_test_large(folio))
> mlock_vma_folio(folio, vma);
> page_vma_mapped_walk_done(&pvmw);
> ret = false;
> break;
> }