[Cc Jack and Hugh - the full patch is http://lkml.kernel.org/r/20190315121826.23609-1-osalvador@xxxxxxx] On Fri 15-03-19 13:18:26, Oscar Salvador wrote: > 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. It is certainly worth fixing. We cannot assume anything about the calling context for __dump_page > We can easily do that by cheking mapping->host first. [...] The splat is still surprising to me because I thought that all file backed mappings have a host. Swap file/partition certainly has a mapping but swapcache mapping is special because the underlying swap storage is hidden in the swap_info_struct. I am wondering whether we should do that special casing for PageSwapCache in __dump_page rather than hid the mapping details instead diff --git a/mm/debug.c b/mm/debug.c index 1611cf00a137..499c26d5ebe5 100644 --- a/mm/debug.c +++ b/mm/debug.c @@ -78,6 +78,9 @@ void __dump_page(struct page *page, const char *reason) else if (PageKsm(page)) pr_warn("ksm "); else if (mapping) { + if (PageSwapCache(page)) + mapping = page_swap_info(page)->swap_file->f_mapping; + pr_warn("%ps ", mapping->a_ops); if (mapping->host->i_dentry.first) { struct dentry *dentry; But I am not really sure this will work for all swap cases. Thanks for reporting this Oscar! > Fixes: 1c6fb1d89e73c ("mm: print more information about mapping in __dump_page") > Signed-off-by: Oscar Salvador <osalvador@xxxxxxx> > --- > mm/debug.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/debug.c b/mm/debug.c > index c0b31b6c3877..7759f12a8fbb 100644 > --- a/mm/debug.c > +++ b/mm/debug.c > @@ -79,7 +79,7 @@ void __dump_page(struct page *page, const char *reason) > 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); > -- > 2.13.7 -- Michal Hocko SUSE Labs
