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

John Hubbard
2020-01-29T22:26:14Z

On 1/29/20 3:25 AM, Kirill A. Shutemov wrote:
> On Tue, Jan 28, 2020 at 07:24:10PM -0800, John Hubbard wrote:
>> 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.
>
> refcount on a tail must always be 0. I think we should only print it when
> it is non-zero, emphasizing this fact with a standalone message.
>

Hi Kirill,

Yes, good point, that sounds like just the right balance. And it avoids adding
a new item to print in the common case, which is very nice. Here's what I've
changed it to for the next version (I'll also rewrite the commit description
accordingly):

diff --git a/mm/debug.c b/mm/debug.c
index a90da5337c14..3a45e2b77de0 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -75,12 +75,17 @@ 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,
+ if (PageCompound(page)) {
+ pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px "
+ "index:%#lx compound_mapcount:%d\n",
+ page, page_ref_count(compound_head(page)), mapcount,
page->mapping, page_to_pgoff(page),
compound_mapcount(page));
+
+ if (page != compound_head(page) && page_ref_count(page) != 0)
+ pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
+ "this tail page\n", page, page_ref_count(page));
+ }
else
pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
page, page_ref_count(page), mapcount,

thanks,
--
John Hubbard
NVIDIA