Re: [PATCHv3 04/24] rmap: add argument to charge compound page

[Vlastimil Babka]

On 03/04/2015 12:52 PM, Kirill A. Shutemov wrote:
> On Mon, Feb 23, 2015 at 05:21:31PM +0100, Vlastimil Babka wrote:
> > On 02/12/2015 05:18 PM, Kirill A. Shutemov wrote:
> > > @@ -1052,21 +1052,24 @@ void page_add_anon_rmap(struct page *page,
> > >    * Everybody else should continue to use page_add_anon_rmap above.
> > >    */
> > >   void do_page_add_anon_rmap(struct page *page,
> > > -	struct vm_area_struct *vma, unsigned long address, int exclusive)
> > > +	struct vm_area_struct *vma, unsigned long address, int flags)
> > >   {
> > >   	int first = atomic_inc_and_test(&page->_mapcount);
> > >   	if (first) {
> > > +		bool compound = flags & RMAP_COMPOUND;
> > > +		int nr = compound ? hpage_nr_pages(page) : 1;
> >
> > hpage_nr_pages(page) is:
> >
> > static inline int hpage_nr_pages(struct page *page)
> > {
> >          if (unlikely(PageTransHuge(page)))
> >                  return HPAGE_PMD_NR;
> >          return 1;
> > }
> >
> > and later...
> >
> > >   		/*
> > >   		 * We use the irq-unsafe __{inc|mod}_zone_page_stat because
> > >   		 * these counters are not modified in interrupt context, and
> > >   		 * pte lock(a spinlock) is held, which implies preemption
> > >   		 * disabled.
> > >   		 */
> > > -		if (PageTransHuge(page))
> > > +		if (compound) {
> > > +			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> >
> > this means that we could assume that
> > (compound == true) => (PageTransHuge(page) == true)
> >
> > and simplify above to:
> >
> > int nr = compound ? HPAGE_PMD_NR : 1;
> >
> > Right?
>
> No. HPAGE_PMD_NR is defined based on HPAGE_PMD_SHIFT which is BUILD_BUG()
> without CONFIG_TRANSPARENT_HUGEPAGE. We will get compiler error without
> the helper.

Oh, OK. But that doesn't mean there couldn't be another helper that 
would work in this case, or even open-coded #ifdefs in these functions. 
Apparently "compound" has to be always false for 
!CONFIG_TRANSPARENT_HUGEPAGE, as in that case PageTransHuge is defined 
as 0 and the VM_BUG_ON would trigger if compound was true. So without 
such ifdefs or wrappers, you are also adding dead code and pointless 
tests for !CONFIG_TRANSPARENT_HUGEPAGE?

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