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Message-ID: <8c1d6e06-d84b-4be7-81c8-76e2d8fb9883@linux.alibaba.com>
Date: Tue, 7 May 2024 17:33:11 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org,
maskray@...gle.com, ziy@...dia.com, ryan.roberts@....com, david@...hat.com,
21cnbao@...il.com, mhocko@...e.com, fengwei.yin@...el.com,
zokeefe@...gle.com, shy828301@...il.com, xiehuan09@...il.com,
libang.li@...group.com, wangkefeng.wang@...wei.com,
songmuchun@...edance.com, peterx@...hat.com, minchan@...nel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] mm/vmscan: avoid split lazyfree THP during
shrink_folio_list()
On 2024/5/7 16:26, Lance Yang wrote:
> On Tue, May 7, 2024 at 2:32 PM Lance Yang <ioworker0@...il.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@...ux.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@...dia.com>
>>>> Suggested-by: David Hildenbrand <david@...hat.com>
>>>> Signed-off-by: Lance Yang <ioworker0@...il.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;
}
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