On 2/3/20 5:16 AM, Kirill A. Shutemov wrote:
On Fri, Jan 31, 2020 at 07:40:18PM -0800, John Hubbard wrote:
A compound page collects the refcount in the head page, while leaving the refcount of each tail page at zero. Therefore, when debugging a problem that involves compound pages, it's best to have diagnostics that reflect that situation. However, dump_page() is oblivious to these points.
Change dump_page() as follows:
For tail pages, print relevant head page information: refcount, in particular. But only do this if the page is not corrupted so badly that the pointer to the head page is all wrong.
Do a separate check to catch any (rare) cases of the tail page's refcount being non-zero, and issue a separate, clear pr_warn() if that ever happens.
Suggested-by: Matthew Wilcox willy@infradead.org Suggested-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Signed-off-by: John Hubbard jhubbard@nvidia.com
Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com
Thanks for looking through all of these!
Few nit-picks below.
mm/debug.c | 34 ++++++++++++++++++++++++++++------ 1 file changed, 28 insertions(+), 6 deletions(-)
diff --git a/mm/debug.c b/mm/debug.c index ecccd9f17801..beb1c59d784b 100644 --- a/mm/debug.c +++ b/mm/debug.c @@ -42,6 +42,32 @@ const struct trace_print_flags vmaflag_names[] = { {0, NULL} }; +static void __dump_tail_page(struct page *page, int mapcount) +{
- struct page *head = compound_head(page);
- if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) {
I'm not sure if we want to use compound_nr() here instead of MAX_ORDER_NR_PAGES. Do you have any reasonaing about it?
Yes: compound_nr(page) reads from the struct page, whereas MAX_ORDER_NR_PAGES is an independent, immutable limit. When checking a struct page for corruption, it's ideal to avoid relying on data within the struct page, as compound_nr() would have to do.
/** Page is hopelessly corrupted, so limit any reporting to* information about the page itself. Do not attempt to look at* the head page.*/pr_warn("page:%px refcount:%d mapcount:%d mapping:%px ""index:%#lx (corrupted tail page case)\n",page, page_ref_count(page), mapcount, page->mapping,page_to_pgoff(page));- } else {
pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px ""index:%#lx compound_mapcount:%d\n",page, page_ref_count(head), mapcount, head->mapping,page_to_pgoff(head), compound_mapcount(page));- }
- if (page_ref_count(page) != 0)
pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on this ""tail page\n", page, page_ref_count(page));Wrap into {}, please.
Fixed, thanks.
thanks,