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

[Song Liu <songliubraving@fb.com>]

> On Aug 12, 2019, at 6:06 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 
> 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.

hmm... how about
		if (pte_none(*pte))
			continue;

		if (!pte_present(*pte))
			return;

If the page hasn't faulted in for this mm, i.e. pte_none(), we
can flush the pmd. 

> 
>> +
>> +		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.

Will update the comment in next version. 

Thanks!
Song