Re: [PATCH uprobe, thp 1/4] mm, thp: allow preallocate pgtable for split_huge_pmd_address()

[Song Liu <songliubraving@fb.com>]

> On May 30, 2019, at 4:14 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> 
> On Thu, May 30, 2019 at 02:10:15PM +0300, Kirill A. Shutemov wrote:
>> On Wed, May 29, 2019 at 02:20:46PM -0700, Song Liu wrote:
>>> @@ -2133,10 +2133,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>> 	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
>>> 	VM_BUG_ON(!is_pmd_migration_entry(*pmd) && !pmd_trans_huge(*pmd)
>>> 				&& !pmd_devmap(*pmd));
>>> +	/* only file backed vma need preallocate pgtable*/
>>> +	VM_BUG_ON(vma_is_anonymous(vma) && prealloc_pgtable);
>>> 
>>> 	count_vm_event(THP_SPLIT_PMD);
>>> 
>>> -	if (!vma_is_anonymous(vma)) {
>>> +	if (prealloc_pgtable) {
>>> +		pgtable_trans_huge_deposit(mm, pmd, prealloc_pgtable);
>>> +		mm_inc_nr_pmds(mm);
>>> +	} else if (!vma_is_anonymous(vma)) {
>>> 		_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
>>> 		/*
>>> 		 * We are going to unmap this huge page. So
>> 
>> Nope. This going to leak a page table for architectures where
>> arch_needs_pgtable_deposit() is true.
> 
> And I don't there's correct handling of dirty bit.
> 
> And what about DAX? Will it blow up? I think so.
> 

Let me look into these cases. Thanks for the feedback!

Song

> -- 
> Kirill A. Shutemov