Re: [PATCH] mm: avoid endless recursion in dump_page()

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Sep 21, 2016 at 04:27:31PM +0200, Vlastimil Babka wrote:
> On 09/08/2016 10:21 AM, Kirill A. Shutemov wrote:
> >dump_page() uses page_mapcount() to get mapcount of the page.
> >page_mapcount() has VM_BUG_ON_PAGE(PageSlab(page)) as mapcount doesn't
> >make sense for slab pages and the field in struct page used for other
> >information.
> >
> >It leads to recursion if dump_page() called for slub page and DEBUG_VM
> >is enabled:
> >
> >dump_page() -> page_mapcount() -> VM_BUG_ON_PAGE() -> dump_page -> ...
> >
> >Let's avoid calling page_mapcount() for slab pages in dump_page().
> 
> How about instead splitting page_mapcount() so that there is a version
> without VM_BUG_ON_PAGE()?

Why? page->_mapping is garbage for slab page and might be confusing.

If you want the information from page->_mapping union for slab page to be
shown during dump_page() we should present in proper way.

> 
> >Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> >---
> > mm/debug.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> >diff --git a/mm/debug.c b/mm/debug.c
> >index 8865bfb41b0b..74c7cae4f683 100644
> >--- a/mm/debug.c
> >+++ b/mm/debug.c
> >@@ -42,9 +42,11 @@ const struct trace_print_flags vmaflag_names[] = {
> >
> > void __dump_page(struct page *page, const char *reason)
> > {
> 
> At least there should be a comment explaining why.

Fair enough.

> >+	int mapcount = PageSlab(page) ? 0 : page_mapcount(page);