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SubjectRe: [PATCH v3 1/2] khugepaged: enable collapse pmd for pte-mapped THP
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> On Aug 2, 2019, at 9:21 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 08/01, Song Liu wrote:
>>
>> +static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
>> + unsigned long addr)
>> +{
>> + 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);
>> + VM_BUG_ON(addr & ~HPAGE_PMD_MASK);
>> +
>> + 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;
>> + }
>
> see my reply to v2
>
>> +void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long haddr)
>> +{
>> + struct vm_area_struct *vma = find_vma(mm, haddr);
>> + pmd_t *pmd = mm_find_pmd(mm, haddr);
>> + struct page *hpage = NULL;
>> + unsigned long addr;
>> + spinlock_t *ptl;
>> + int count = 0;
>> + pmd_t _pmd;
>> + int i;
>> +
>> + VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
>> +
>> + if (!vma || !vma->vm_file || !pmd)
> ^^^^^^^^^^^^^
>
> I am not sure this is enough,
>
>> + return;
>> +
>> + /* step 1: check all mapped PTEs are to the right huge page */
>> + for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
>> + pte_t *pte = pte_offset_map(pmd, addr);
>> + struct page *page;
>> +
>> + if (pte_none(*pte))
>> + continue;
>> +
>> + page = vm_normal_page(vma, addr, *pte);
>
> Why can't vm_normal_page() return NULL? Again, we do not if this vm_file
> is the same shmem_file() or something else.

Hmm.. I guess we also need to check page != NULL.

For vm_file, I guess it is OK for it to be another file. As long as all
pages in the pmd map to the same THP, we should be fine.

>
> And in fact I don't think it is safe to use vm_normal_page(vma, addr)
> unless you know that vma includes this addr.

Yeah, we need to check vma includes this address.

>
> to be honest, I am not even sure that unconditional mm_find_pmd() is safe
> if this "something else" is really special.

I cannot imagine why mm_find_pmd() could be unsafe.

Thanks,
Song

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