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SubjectRe: [PATCH 1/5] mm/debug_pagealloc: clean-up guard page handling code
On (08/11/16 11:41), Vlastimil Babka wrote:
> On 08/10/2016 10:14 AM, Sergey Senozhatsky wrote:
> > > @@ -1650,18 +1655,15 @@ static inline void expand(struct zone *zone, struct page *page,
> > > size >>= 1;
> > > VM_BUG_ON_PAGE(bad_range(zone, &page[size]), &page[size]);
> > >
> > > - if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC) &&
> > > - debug_guardpage_enabled() &&
> > > - high < debug_guardpage_minorder()) {
> > > - /*
> > > - * Mark as guard pages (or page), that will allow to
> > > - * merge back to allocator when buddy will be freed.
> > > - * Corresponding page table entries will not be touched,
> > > - * pages will stay not present in virtual address space
> > > - */
> > > - set_page_guard(zone, &page[size], high, migratetype);
> > > + /*
> > > + * Mark as guard pages (or page), that will allow to
> > > + * merge back to allocator when buddy will be freed.
> > > + * Corresponding page table entries will not be touched,
> > > + * pages will stay not present in virtual address space
> > > + */
> > > + if (set_page_guard(zone, &page[size], high, migratetype))
> > > continue;
> > > - }
> >
> > so previously IS_ENABLED(CONFIG_DEBUG_PAGEALLOC) could have optimized out
> > the entire branch -- no set_page_guard() invocation and checks, right? but
> > now we would call set_page_guard() every time?
>
> No, there's a !CONFIG_DEBUG_PAGEALLOC version of set_page_guard() that
> returns false (static inline), so this whole if will be eliminated by the
> compiler, same as before.

ah, indeed. didn't notice it.

-ss

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