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