linux-kernel - Re: [PATCH] hugetlb: Do not account hugetlb pages as NR_FILE

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Date:	Tue, 2 Jun 2015 11:25:35 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hugetlb: Do not account hugetlb pages as NR_FILE_PAGES

On Mon 25-05-15 17:24:28, Vlastimil Babka wrote:
> On 05/22/2015 04:35 PM, Mel Gorman wrote:
> >> 
> >> Thanks!
> >> 
> >> > This makes a lot of sense to me.  The only thing I worry about is the
> >> > proliferation of PageHuge(), a function call, in relatively hot paths.
> >> 
> >> I've tried that (see the patch below) but it enlarged the code by almost
> >> 1k
> >>    text    data     bss     dec     hex filename
> >>  510323   74273   44440  629036   9992c mm/built-in.o.before
> >>  511248   74273   44440  629961   99cc9 mm/built-in.o.after
> >> 
> >> I am not sure the code size increase is worth it. Maybe we can reduce
> >> the check to only PageCompound(page) as huge pages are no in the page
> >> cache (yet).
> >> 
> > 
> > That would be a more sensible route because it also avoids exposing the
> > hugetlbfs destructor unnecessarily.
> 
> You could maybe do test such as (PageCompound(page) && PageHuge(page)) to
> short-circuit the call while remaining future-proof.

How about this?
---
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 91b7f9b2b774..bb8a70e8fc77 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -547,7 +547,13 @@ static inline void ClearPageCompound(struct page *page)
 #endif /* !PAGEFLAGS_EXTENDED */
 
 #ifdef CONFIG_HUGETLB_PAGE
-int PageHuge(struct page *page);
+int __PageHuge(struct page *page);
+static inline int PageHuge(struct page *page)
+{
+	if (!PageCompound(page))
+		return 0;
+	return __PageHuge(page);
+}
 int PageHeadHuge(struct page *page);
 bool page_huge_active(struct page *page);
 #else
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 33defbe1897f..648c0c32857c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1107,19 +1107,17 @@ static void prep_compound_gigantic_page(struct page *page, unsigned long order)
 }
 
 /*
- * PageHuge() only returns true for hugetlbfs pages, but not for normal or
+ * __PageHuge() only returns true for hugetlbfs pages, but not for normal or
  * transparent huge pages.  See the PageTransHuge() documentation for more
  * details.
  */
-int PageHuge(struct page *page)
+int __PageHuge(struct page *page)
 {
-	if (!PageCompound(page))
-		return 0;
-
+	VM_BUG_ON(!PageCompound(page));
 	page = compound_head(page);
 	return get_compound_page_dtor(page) == free_huge_page;
 }
-EXPORT_SYMBOL_GPL(PageHuge);
+EXPORT_SYMBOL_GPL(__PageHuge);
 
 /*
  * PageHeadHuge() only returns true for hugetlbfs head page, but not for

-- 
Michal Hocko
SUSE Labs
--
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