From: Oscar Salvador osalvador@suse.de Subject: mm/debug.c: fix __dump_page when mapping->host is not set

While debugging something, I added a dump_page() into do_swap_page(), and I got the splat from below. The issue happens when dereferencing mapping->host in __dump_page():

... else if (mapping) { pr_warn("%ps ", mapping->a_ops); if (mapping->host->i_dentry.first) { struct dentry *dentry; dentry = container_of(mapping->host->i_dentry.first, struct dentry, d_u.d_alias); pr_warn("name:"%pd" ", dentry); } } ...

Swap address space does not contain an inode information, and so mapping->host equals NULL.

Although the dump_page() call was added artificially into do_swap_page(), I am not sure if we can hit this from any other path, so it looks worth fixing it. We can easily do that by checking mapping->host first.

Link: http://lkml.kernel.org/r/20190318072931.29094-1-osalvador@suse.de Fixes: 1c6fb1d89e73c ("mm: print more information about mapping in __dump_page") Signed-off-by: Oscar Salvador osalvador@suse.de Acked-by: Michal Hocko mhocko@suse.com Acked-by: Hugh Dickins hughd@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/debug.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/debug.c~mm-fix-__dump_page-when-mapping-host-is-not-set +++ a/mm/debug.c @@ -79,7 +79,7 @@ void __dump_page(struct page *page, cons pr_warn("ksm "); else if (mapping) { pr_warn("%ps ", mapping->a_ops); - if (mapping->host->i_dentry.first) { + if (mapping->host && mapping->host->i_dentry.first) { struct dentry *dentry; dentry = container_of(mapping->host->i_dentry.first, struct dentry, d_u.d_alias); pr_warn("name:"%pd" ", dentry); _