On Thu, Apr 02, 2020 at 10:30:35AM +0200, Vlastimil Babka wrote: > On 4/1/20 4:05 PM, Kirill A. Shutemov wrote: > > On Tue, Mar 31, 2020 at 06:54:54PM +0200, Vlastimil Babka wrote: > >> We have seen a following problem on a RPi4 with 1G RAM: > >> > >> Besides the underlying issue with page->mapping containing a bogus value for > >> some reason, we can see that __dump_page() crashed by trying to read the > >> pointer at mapping->host, turning a recoverable warning into full Oops. > >> > >> It can be expected that when page is reported as bad state for some reason, the > >> pointers there should not be trusted blindly. So this patch treats all data in > >> __dump_page() that depends on page->mapping as lava, using > >> probe_kernel_read_strict(). Ideally this would include the dentry->d_parent > >> recursively, but that would mean changing printk handler for %pd. Chances of > >> reaching the dentry printing part with an initially bogus mapping pointer > >> should be rather low, though. > >> > >> Also prefix printing mapping->a_ops with a description of what is being > >> printed. In case the value is bogus, %ps will print raw value instead of > >> the symbol name and then it's not obvious at all that it's printing a_ops. > >> > >> Reported-by: Petr Tesarik <ptesarik@xxxxxxx> > >> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > >> --- > >> mm/debug.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++------ > >> 1 file changed, 50 insertions(+), 6 deletions(-) > > > > I'm not sure it worth the effort. It looks way too complex for what it > > does. > > Well the human effort is done, and CPU cycles are cheap :P Complex is better > than to crash, IMHO. > > > I also expect it to slowdown dump_page(), which is hotpath for some debug > > scenarios :P > > It's still a debug code, better safe than fast :P Crash fast, crash often :P > > Maybe just move printing this info to the end, so we would see the rest > > even if ->mapping is bogus? > > Well the thing is designed to be recoverable. Just today "mm: improve > dump_page() for compound pages" was merged that AFAICS prevents similar crashes > when the compound_head() is bogus. Okay, fair enough. Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> -- Kirill A. Shutemov
