Re: [RFC 1/7] mm: introduce MADV_COOL

[Minchan Kim <minchan@kernel.org>]

On Tue, May 28, 2019 at 04:53:01PM +0800, Hillf Danton wrote:
> 
> On Mon, 20 May 2019 12:52:48 +0900 Minchan Kim wrote:
> > +static int madvise_cool_pte_range(pmd_t *pmd, unsigned long addr,
> > +				unsigned long end, struct mm_walk *walk)
> > +{
> > +	pte_t *orig_pte, *pte, ptent;
> > +	spinlock_t *ptl;
> > +	struct page *page;
> > +	struct vm_area_struct *vma = walk->vma;
> > +	unsigned long next;
> > +
> > +	next = pmd_addr_end(addr, end);
> > +	if (pmd_trans_huge(*pmd)) {
> > +		spinlock_t *ptl;
> 
> Seems not needed with another ptl declared above.

Will remove it.

> > +
> > +		ptl = pmd_trans_huge_lock(pmd, vma);
> > +		if (!ptl)
> > +			return 0;
> > +
> > +		if (is_huge_zero_pmd(*pmd))
> > +			goto huge_unlock;
> > +
> > +		page = pmd_page(*pmd);
> > +		if (page_mapcount(page) > 1)
> > +			goto huge_unlock;
> > +
> > +		if (next - addr != HPAGE_PMD_SIZE) {
> > +			int err;
> 
> Alternately, we deactivate thp only if the address range from userspace
> is sane enough, in order to avoid complex works we have to do here.

Not sure it's a good idea. That's the way we have done in MADV_FREE
so want to be consistent.

> > +
> > +			get_page(page);
> > +			spin_unlock(ptl);
> > +			lock_page(page);
> > +			err = split_huge_page(page);
> > +			unlock_page(page);
> > +			put_page(page);
> > +			if (!err)
> > +				goto regular_page;
> > +			return 0;
> > +		}
> > +
> > +		pmdp_test_and_clear_young(vma, addr, pmd);
> > +		deactivate_page(page);
> > +huge_unlock:
> > +		spin_unlock(ptl);
> > +		return 0;
> > +	}
> > +
> > +	if (pmd_trans_unstable(pmd))
> > +		return 0;
> > +
> > +regular_page:
> 
> Take a look at pending signal?

Do you have any reason to see pending signal here? I want to know what's
your requirement so that what's the better place to handle it.

> 
> > +	orig_pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> > +	for (pte = orig_pte; addr < end; pte++, addr += PAGE_SIZE) {
> 
> s/end/next/ ?

Why do you think it should be next?

> > +		ptent = *pte;
> > +
> > +		if (pte_none(ptent))
> > +			continue;
> > +
> > +		if (!pte_present(ptent))
> > +			continue;
> > +
> > +		page = vm_normal_page(vma, addr, ptent);
> > +		if (!page)
> > +			continue;
> > +
> > +		if (page_mapcount(page) > 1)
> > +			continue;
> > +
> > +		ptep_test_and_clear_young(vma, addr, pte);
> > +		deactivate_page(page);
> > +	}
> > +
> > +	pte_unmap_unlock(orig_pte, ptl);
> > +	cond_resched();
> > +
> > +	return 0;
> > +}
> > +
> > +static long madvise_cool(struct vm_area_struct *vma,
> > +			unsigned long start_addr, unsigned long end_addr)
> > +{
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	struct mmu_gather tlb;
> > +
> > +	if (vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP))
> > +		return -EINVAL;
> 
> No service in case of VM_IO?

I don't know VM_IO would have regular LRU pages but just follow normal
convention for DONTNEED and FREE.
Do you have anything in your mind?