On Wed, Jan 29, 2020 at 02:26:06PM -0800, John Hubbard wrote:
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,
I have a hunk in my current tree which looks like this:
@@ -77,6 +77,11 @@ void __dump_page(struct page *page, const char *reason) pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx \n", page, page_ref_count(page), mapcount, page->mapping, page_to_pgoff(page)); + if (PageTail(page)) { + struct page *head = compound_head(page); + pr_warn("head:%px mapping:%px index:%#lx\n", + head, head->mapping, page_to_pgoff(head)); + } if (PageKsm(page)) pr_warn("ksm flags: %#lx(%pGp)\n", page->flags, &page->flags); else if (PageAnon(page))
I wonder if we can combine these two patches in some more useful way?
I also think we probably want a sanity check that 'head' and 'page' are within a sane range of each other (ie head < page and head + MAX_ORDER_NR_PAGES > page) to protect against a struct page that contains complete garbage.