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

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.