[PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages

John Hubbard
2020-01-29T03:24:24Z

When debugging a problem that involves compound pages, it is extremely
helpful if dump_page() reports not only the page->_refcount, but also
the refcount of the head page of the compound page. That's because the
head page collects refcounts for the entire compound page.

Therefore, enhance dump_page() so as to print out the refcount of the
head page of a compound page.

This approach (printing information about a struct page that is not the
struct page that was passed into dump_page()) has a precedent:
compound_mapcount is already being printed.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
mm/debug.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index a90da5337c14..4cc6cad8385d 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -76,9 +76,11 @@ void __dump_page(struct page *page, const char *reason)
mapcount = PageSlab(page) ? 0 : page_mapcount(page);

if (PageCompound(page))
- pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
- "index:%#lx compound_mapcount: %d\n",
- page, page_ref_count(page), mapcount,
+ pr_warn("page:%px refcount:%d head refcount:%d "
+ "mapcount:%d mapping:%px index:%#lx "
+ "compound_mapcount:%d\n",
+ page, page_ref_count(page),
+ page_ref_count(compound_head(page)), mapcount,
page->mapping, page_to_pgoff(page),
compound_mapcount(page));
else
--
2.25.0