Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP

[Oleg Nesterov <oleg@redhat.com>]

On 08/09, Song Liu wrote:
>
> +void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
> +{
> +	unsigned long haddr = addr & HPAGE_PMD_MASK;
> +	struct vm_area_struct *vma = find_vma(mm, haddr);
> +	struct page *hpage = NULL;
> +	pmd_t *pmd, _pmd;
> +	spinlock_t *ptl;
> +	int count = 0;
> +	int i;
> +
> +	if (!vma || !vma->vm_file ||
> +	    vma->vm_start > haddr || vma->vm_end < haddr + HPAGE_PMD_SIZE)
> +		return;
> +
> +	/*
> +	 * This vm_flags may not have VM_HUGEPAGE if the page was not
> +	 * collapsed by this mm. But we can still collapse if the page is
> +	 * the valid THP. Add extra VM_HUGEPAGE so hugepage_vma_check()
> +	 * will not fail the vma for missing VM_HUGEPAGE
> +	 */
> +	if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
> +		return;
> +
> +	pmd = mm_find_pmd(mm, haddr);
> +	if (!pmd)
> +		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) || !pte_present(*pte))
> +			continue;

		if (!pte_present(*pte))
			return;

you can't simply flush pmd if this page is swapped out.

> +
> +		page = vm_normal_page(vma, addr, *pte);
> +
> +		if (!page || !PageCompound(page))
> +			return;
> +
> +		if (!hpage) {
> +			hpage = compound_head(page);
> +			/*
> +			 * The mapping of the THP should not change.
> +			 *
> +			 * Note that uprobe may change the page table,

Not only uprobe can cow the page. Debugger can do. Or mmap(PROT_WRITE, MAP_PRIVATE).

uprobe() is "special" because it a) it works with a foreign mm and b)
it can't stop the process which uses this mm. Otherwise it could simply
update the page returned by get_user_pages_remote(FOLL_FORCE), just we
would need to add FOLL_WRITE and if we do this we do not even need SPLIT,
that is why, say, __access_remote_vm() works without SPLIT.

Oleg.