Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page

[Aneesh Kumar K.V]

"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> writes:

> Current split_huge_page() combines two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> changes split_huge_pmd() implementation to split the given PMD without
> splitting other PMDs this page mapped with or underlying compound page.
>
> In order to do this we have to get rid of tail page refcounting, which
> uses _mapcount of tail pages. Tail page refcounting is needed to be able
> to split THP page at any point: we always know which of tail pages is
> pinned (i.e. by get_user_pages()) and can distribute page count
> correctly.
>
> We can avoid this by allowing split_huge_page() to fail if the compound
> page is pinned. This patch removes all infrastructure for tail page
> refcounting and make split_huge_page() to always return -EBUSY. All
> split_huge_page() users already know how to handle its fail. Proper
> implementation will be added later.
>
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.
>
> Memory cgroup is not yet ready for new refcouting. Let's disable it on
> Kconfig level.
>
.....
......

>
>  	spin_lock(ptl);
>  	if (page)
> -		put_user_huge_page(page);
> +		put_page(page);
>  	if (unlikely(!pmd_same(*pmd, orig_pmd))) {
>  		spin_unlock(ptl);
>  		mem_cgroup_cancel_charge(new_page, memcg);
> @@ -1662,51 +1631,78 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
>  	put_huge_zero_page();
>  }
>
> -void __split_huge_page_pmd(struct vm_area_struct *vma, unsigned long address,
> -		pmd_t *pmd)
> +
> +static void __split_huge_pmd_locked(struct vm_area_struct *vma,
> +		pmd_t *pmd, unsigned long address)
>  {
> -	spinlock_t *ptl;
> +	unsigned long haddr = address & HPAGE_PMD_MASK;
>  	struct page *page;
>  	struct mm_struct *mm = vma->vm_mm;
> -	unsigned long haddr = address & HPAGE_PMD_MASK;
> -	unsigned long mmun_start;	/* For mmu_notifiers */
> -	unsigned long mmun_end;		/* For mmu_notifiers */
> +	pgtable_t pgtable;
> +	pmd_t _pmd;
> +	bool young, write;
> +	int i;
>
> -	BUG_ON(vma->vm_start > haddr || vma->vm_end < haddr + HPAGE_PMD_SIZE);
> +	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
> +	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
> +
> +	if (is_huge_zero_pmd(*pmd))
> +		return __split_huge_zero_page_pmd(vma, haddr, pmd);
> +
> +	page = pmd_page(*pmd);
> +	VM_BUG_ON_PAGE(!page_count(page), page);
> +	atomic_add(HPAGE_PMD_NR - 1, &page->_count);
> +
> +	write = pmd_write(*pmd);
> +	young = pmd_young(*pmd);
> +
> +	/* leave pmd empty until pte is filled */
> +	pmdp_clear_flush_notify(vma, haddr, pmd);
> +

So we now mark pmd none, while we go ahead and split the pmd. But then what
happens to a parallel fault ? We don't hold mmap_sem here right ?

> +	pgtable = pgtable_trans_huge_withdraw(mm, pmd);
> +	pmd_populate(mm, &_pmd, pgtable);
> +
> +	for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
> +		pte_t entry, *pte;
> +		/*
> +		 * Note that NUMA hinting access restrictions are not
> +		 * transferred to avoid any possibility of altering
> +		 * permissions across VMAs.
> +		 */
> +		entry = mk_pte(page + i, vma->vm_page_prot);
> +		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +		if (!write)
> +			entry = pte_wrprotect(entry);
> +		if (!young)
> +			entry = pte_mkold(entry);
> +		pte = pte_offset_map(&_pmd, haddr);
> +		BUG_ON(!pte_none(*pte));
> +		atomic_inc(&page[i]._mapcount);
> +		set_pte_at(mm, haddr, pte, entry);
> +		pte_unmap(pte);
> +	}
> +	smp_wmb(); /* make pte visible before pmd */
> +	pmd_populate(mm, pmd, pgtable);
> +	atomic_dec(compound_mapcount_ptr(page));
> +}
> +

-aneesh

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