Re: [PATCH 17/19] mlock, thp: HACK: split all pages in VM_LOCKED vma

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Nov 19, 2014 at 09:02:42AM +0000, Naoya Horiguchi wrote:
> On Wed, Nov 05, 2014 at 04:49:52PM +0200, Kirill A. Shutemov wrote:
> > We don't yet handle mlocked pages properly with new THP refcounting.
> > For now we split all pages in VMA on mlock and disallow khugepaged
> > collapse pages in the VMA. If split failed on mlock() we fail the
> > syscall with -EBUSY.
> > ---
> ...
> 
> > @@ -542,6 +530,60 @@ next:
> >  	}
> >  }
> >  
> > +static int thp_split(pmd_t *pmd, unsigned long addr, unsigned long end,
> > +		struct mm_walk *walk)
> > +{
> > +	spinlock_t *ptl;
> > +	struct page *page = NULL;
> > +	pte_t *pte;
> > +	int err = 0;
> > +
> > +retry:
> > +	if (pmd_none(*pmd))
> > +		return 0;
> > +	if (pmd_trans_huge(*pmd)) {
> > +		if (is_huge_zero_pmd(*pmd)) {
> > +			split_huge_pmd(walk->vma, pmd, addr);
> > +			return 0;
> > +		}
> > +		ptl = pmd_lock(walk->mm, pmd);
> > +		if (!pmd_trans_huge(*pmd)) {
> > +			spin_unlock(ptl);
> > +			goto retry;
> > +		}
> > +		page = pmd_page(*pmd);
> > +		VM_BUG_ON_PAGE(!PageHead(page), page);
> > +		get_page(page);
> > +		spin_unlock(ptl);
> > +		err = split_huge_page(page);
> > +		put_page(page);
> > +		return err;
> > +	}
> > +	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> > +	do {
> > +		if (!pte_present(*pte))
> > +			continue;
> > +		page = vm_normal_page(walk->vma, addr, *pte);
> > +		if (!page)
> > +			continue;
> > +		if (PageTransCompound(page)) {
> > +			page = compound_head(page);
> > +			get_page(page);
> > +			spin_unlock(ptl);
> > +			err = split_huge_page(page);
> > +			spin_lock(ptl);
> > +			put_page(page);
> > +			if (!err) {
> > +				VM_BUG_ON_PAGE(compound_mapcount(page), page);
> > +				VM_BUG_ON_PAGE(PageTransCompound(page), page);
> 
> If split_huge_page() succeeded, we don't have to continue the iteration,
> so break this loop here?

We may want to skip to the next huge page region, but the patch is crap
anyway -- don't bother.

-- 
 Kirill A. Shutemov