Re: 6.6.8 stable: crash in folio_mark_dirty

Matthew Wilcox
2024-01-01T09:08:10Z

On Mon, Jan 01, 2024 at 09:55:04AM +0800, Hillf Danton wrote:
> On Sun, 31 Dec 2023 13:07:03 +0000 Matthew Wilcox <willy@infradead.org>
> > On Sun, Dec 31, 2023 at 09:28:46AM +0800, Hillf Danton wrote:
> > > On Sat, Dec 30, 2023 at 10:23:26AM -0500 Genes Lists <lists@sapience.com>
> > > > Apologies in advance, but I cannot git bisect this since machine was
> > > > running for 10 days on 6.6.8 before this happened.
> > > >
> > > > Dec 30 07:00:36 s6 kernel: ------------[ cut here ]------------
> > > > Dec 30 07:00:36 s6 kernel: WARNING: CPU: 0 PID: 521524 at mm/page-writeback.c:2668 __folio_mark_dirty (??:?)
> > > > Dec 30 07:00:36 s6 kernel: CPU: 0 PID: 521524 Comm: rsync Not tainted 6.6.8-stable-1 #13 d238f5ab6a206cdb0cc5cd72f8688230f23d58df
> > > > Dec 30 07:00:36 s6 kernel: block_dirty_folio (??:?)
> > > > Dec 30 07:00:36 s6 kernel: unmap_page_range (??:?)
> > > > Dec 30 07:00:36 s6 kernel: unmap_vmas (??:?)
> > > > Dec 30 07:00:36 s6 kernel: exit_mmap (??:?)
> > > > Dec 30 07:00:36 s6 kernel: __mmput (??:?)
> > > > Dec 30 07:00:36 s6 kernel: do_exit (??:?)
> > > > Dec 30 07:00:36 s6 kernel: do_group_exit (??:?)
> > > > Dec 30 07:00:36 s6 kernel: __x64_sys_exit_group (??:?)
> > > > Dec 30 07:00:36 s6 kernel: do_syscall_64 (??:?)
> > >
> > > See what comes out if race is handled.
> > > Only for thoughts.
> >
> > I don't think this can happen. Look at the call trace;
> > block_dirty_folio() is called from unmap_page_range(). That means the
> > page is in the page tables. We unmap the pages in a folio from the
> > page tables before we set folio->mapping to NULL. Look at
> > invalidate_inode_pages2_range() for example:
> >
> > unmap_mapping_pages(mapping, indices[i],
> > (1 + end - indices[i]), false);
> > folio_lock(folio);
> > folio_wait_writeback(folio);
> > if (folio_mapped(folio))
> > unmap_mapping_folio(folio);
> > BUG_ON(folio_mapped(folio));
> > if (!invalidate_complete_folio2(mapping, folio))
> >
> What is missed here is the same check [1] in invalidate_inode_pages2_range(),
> so I built no wheel.
>
> folio_lock(folio);
> if (unlikely(folio->mapping != mapping)) {
> folio_unlock(folio);
> continue;
> }
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/truncate.c#n658

That's entirely different. That's checking in the truncate path whether
somebody else already truncated this page. What I was showing was why
a page found through a page table walk cannot have been truncated (which
is actually quite interesting, because it's the page table lock that
prevents the race).