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Date: Tue, 30 Jul 2019 18:39:54 +0000
From: Song Liu <songliubraving@...com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
CC: lkml <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
"oleg@...hat.com" <oleg@...hat.com>,
Kernel Team <Kernel-team@...com>,
"william.kucharski@...cle.com" <william.kucharski@...cle.com>,
"srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] khugepaged: enable collapse pmd for pte-mapped THP
> On Jul 30, 2019, at 10:28 AM, Song Liu <songliubraving@...com> wrote:
>
>
>
>> On Jul 30, 2019, at 7:59 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
>>
>> On Sun, Jul 28, 2019 at 10:43:34PM -0700, Song Liu wrote:
>>> khugepaged needs exclusive mmap_sem to access page table. When it fails
>>> to lock mmap_sem, the page will fault in as pte-mapped THP. As the page
>>> is already a THP, khugepaged will not handle this pmd again.
>>>
>>> This patch enables the khugepaged to retry collapse the page table.
>>>
>>> struct mm_slot (in khugepaged.c) is extended with an array, containing
>>> addresses of pte-mapped THPs. We use array here for simplicity. We can
>>> easily replace it with more advanced data structures when needed. This
>>> array is protected by khugepaged_mm_lock.
>>>
>>> In khugepaged_scan_mm_slot(), if the mm contains pte-mapped THP, we try
>>> to collapse the page table.
>>>
>>> Since collapse may happen at an later time, some pages may already fault
>>> in. collapse_pte_mapped_thp() is added to properly handle these pages.
>>> collapse_pte_mapped_thp() also double checks whether all ptes in this pmd
>>> are mapping to the same THP. This is necessary because some subpage of
>>> the THP may be replaced, for example by uprobe. In such cases, it is not
>>> possible to collapse the pmd.
>>>
>>> Signed-off-by: Song Liu <songliubraving@...com>
>>> ---
>>> include/linux/khugepaged.h | 15 ++++
>>> mm/khugepaged.c | 136 +++++++++++++++++++++++++++++++++++++
>>> 2 files changed, 151 insertions(+)
>>>
>>> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
>>> index 082d1d2a5216..2d700830fe0e 100644
>>> --- a/include/linux/khugepaged.h
>>> +++ b/include/linux/khugepaged.h
>>> @@ -15,6 +15,16 @@ extern int __khugepaged_enter(struct mm_struct *mm);
>>> extern void __khugepaged_exit(struct mm_struct *mm);
>>> extern int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
>>> unsigned long vm_flags);
>>> +#ifdef CONFIG_SHMEM
>>> +extern int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
>>> + unsigned long addr);
>>> +#else
>>> +static inline int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
>>> + unsigned long addr)
>>> +{
>>> + return 0;
>>> +}
>>> +#endif
>>>
>>> #define khugepaged_enabled() \
>>> (transparent_hugepage_flags & \
>>> @@ -73,6 +83,11 @@ static inline int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
>>> {
>>> return 0;
>>> }
>>> +static inline int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
>>> + unsigned long addr)
>>> +{
>>> + return 0;
>>> +}
>>> #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>>>
>>> #endif /* _LINUX_KHUGEPAGED_H */
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index eaaa21b23215..247c25aeb096 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -76,6 +76,7 @@ static __read_mostly DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
>>>
>>> static struct kmem_cache *mm_slot_cache __read_mostly;
>>>
>>> +#define MAX_PTE_MAPPED_THP 8
>>
>> Is MAX_PTE_MAPPED_THP value random or do you have any justification for
>> it?
>
> In our use cases, we only have small number (< 10) of huge pages for the
> text section, so 8 should be enough to cover the worse case.
>
> If this is not sufficient, we can make it a list.
>
>>
>> Please add empty line after it.
>>
>>> /**
>>> * struct mm_slot - hash lookup from mm to mm_slot
>>> * @hash: hash collision list
>>> @@ -86,6 +87,10 @@ struct mm_slot {
>>> struct hlist_node hash;
>>> struct list_head mm_node;
>>> struct mm_struct *mm;
>>> +
>>> + /* pte-mapped THP in this mm */
>>> + int nr_pte_mapped_thp;
>>> + unsigned long pte_mapped_thp[MAX_PTE_MAPPED_THP];
>>> };
>>>
>>> /**
>>> @@ -1281,11 +1286,141 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
>>> up_write(&vma->vm_mm->mmap_sem);
>>> mm_dec_nr_ptes(vma->vm_mm);
>>> pte_free(vma->vm_mm, pmd_pgtable(_pmd));
>>> + } else if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
>>> + /* need down_read for khugepaged_test_exit() */
>>> + khugepaged_add_pte_mapped_thp(vma->vm_mm, addr);
>>> + up_read(&vma->vm_mm->mmap_sem);
>>> }
>>> }
>>> i_mmap_unlock_write(mapping);
>>> }
>>>
>>> +/*
>>> + * Notify khugepaged that given addr of the mm is pte-mapped THP. Then
>>> + * khugepaged should try to collapse the page table.
>>> + */
>>> +int khugepaged_add_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
>>
>> What is contract about addr alignment? Do we expect it PAGE_SIZE aligned
>> or PMD_SIZE aligned? Do we want to enforce it?
>
> It is PMD_SIZE aligned. Let me add VM_BUG_ON() for it.
>
>>
>>> +{
>>> + struct mm_slot *mm_slot;
>>> + int ret = 0;
>>> +
>>> + /* hold mmap_sem for khugepaged_test_exit() */
>>> + VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
>>> +
>>> + if (unlikely(khugepaged_test_exit(mm)))
>>> + return 0;
>>> +
>>> + if (!test_bit(MMF_VM_HUGEPAGE, &mm->flags) &&
>>> + !test_bit(MMF_DISABLE_THP, &mm->flags)) {
>>> + ret = __khugepaged_enter(mm);
>>> + if (ret)
>>> + return ret;
>>> + }
>>
>> Any reason not to call khugepaged_enter() here?
>
> No specific reasons... Let me try it.
Actually, khugepaged_enter() takes vma and vm_flags; while here we only
have the mm. I guess we should just use __khugepaged_enter(). Once we
remove all checks on vm_flags, khugepaged_enter() is about the same as
the logic above.
Thanks,
Song
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