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

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

On Sun, Mar 29, 2015 at 09:25:29PM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> 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.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> .....
> .....
> 
> >  static inline int page_mapped(struct page *page)
> >  {
> > -	return atomic_read(&(page)->_mapcount) + compound_mapcount(page) >= 0;
> > +	int i;
> > +	if (likely(!PageCompound(page)))
> > +		return atomic_read(&page->_mapcount) >= 0;
> > +	if (compound_mapcount(page))
> > +		return 1;
> > +	for (i = 0; i < hpage_nr_pages(page); i++) {
> > +		if (atomic_read(&page[i]._mapcount) >= 0)
> > +			return 1;
> > +	}
> 
> do we need to loop with head page here ? ie,

We do need to loop if we have only tail pages mapped. Partial unmap case.

> 
> page = compound_page(page);

compound_head() ?

This function expects to see head page on input. I'll put VM_BUG_ON()
there to make sure we don't call it tail pages.

> > +	return 0;
> >  }

-- 
 Kirill A. Shutemov