[patch 096/127] mm: clarify why we avoid page_mapcount() for slab pages in dump_page()

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From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Subject: mm: clarify why we avoid page_mapcount() for slab pages in dump_page()

Let's add comment on why we skip page_mapcount() for sl[aou]b pages.

Link: http://lkml.kernel.org/r/20160922105532.GB24593@node
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/debug.c |    5 +++++
 1 file changed, 5 insertions(+)

diff -puN mm/debug.c~mm-clarify-why-we-avoid-page_mapcount-for-slab-pages-in-dump_page mm/debug.c
--- a/mm/debug.c~mm-clarify-why-we-avoid-page_mapcount-for-slab-pages-in-dump_page
+++ a/mm/debug.c
@@ -42,6 +42,11 @@ const struct trace_print_flags vmaflag_n
 
 void __dump_page(struct page *page, const char *reason)
 {
+	/*
+	 * Avoid VM_BUG_ON() in page_mapcount().
+	 * page->_mapcount space in struct page is used by sl[aou]b pages to
+	 * encode own info.
+	 */
 	int mapcount = PageSlab(page) ? 0 : page_mapcount(page);
 
 	pr_emerg("page:%p count:%d mapcount:%d mapping:%p index:%#lx",
_
--
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