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

Kirill A. Shutemov
2015-03-04T13:26:43Z

On Wed, Mar 04, 2015 at 01:09:51PM +0100, Vlastimil Babka wrote:
> 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?

Yeah, this definitely can be improved. I prefer to do it as follow up.
I want to get stable what I have now.

--
Kirill A. Shutemov