Re: [f2fs-dev] [syzbot] BUG: unable to handle kernel NULL pointer dereference in folio_mark_dirty

[Chao Yu]

On 2021/12/8 6:10, Jaegeuk Kim wrote:
> On 12/07, Matthew Wilcox wrote:
> > On Tue, Dec 07, 2021 at 01:39:06PM -0800, Jaegeuk Kim wrote:
> > > On 12/07, Matthew Wilcox wrote:
> > > > > > Call Trace:
> > > > > >   <TASK>
> > > > > >   folio_mark_dirty+0x136/0x270 mm/page-writeback.c:2639
> > > >
> > > >          if (likely(mapping)) {
> > > > ...
> > > >                  if (folio_test_reclaim(folio))
> > > >                          folio_clear_reclaim(folio);
> > > >                  return mapping->a_ops->set_page_dirty(&folio->page);
> > > >
> > > > how do we get to a NULL ->set_page_dirty for a metadata page's
> > > > mapping->a_ops?  This is definitely an f2fs expert question.
> > >
> > > I can't find anything in f2fs, since that page was got by f2fs_grab_meta_page
> > > along with grab_cache_page() that we never unlocked it.
> > >
> > >    40 struct page *f2fs_grab_meta_page(struct f2fs_sb_info *sbi, pgoff_t index)
> > >    41 {
> > >    42         struct address_space *mapping = META_MAPPING(sbi);
> > >    43         struct page *page;
> > >    44 repeat:
> > >    45         page = f2fs_grab_cache_page(mapping, index, false);
> > >
> > >                      -> grab_cache_page(mapping, index);
> > >
> > >    46         if (!page) {
> > >    47                 cond_resched();
> > >    48                 goto repeat;
> > >    49         }
> > >    50         f2fs_wait_on_page_writeback(page, META, true, true);
> > >    51         if (!PageUptodate(page))
> > >    52                 SetPageUptodate(page);
> > >    53         return page;
> > >    54 }
> > >
> > >
> > > Suspecting something in folio wrt folio_mapping()?
> > >
> > >   81 bool set_page_dirty(struct page *page)
> > >   82 {
> > >   83         return folio_mark_dirty(page_folio(page));
> > >   84 }
> >
> > ... huh?  How could folio_mapping() be getting this wrong?
>
> Dunno.
>
> > page_folio() does the same thing as compound_head() -- as far as I know
> > you don't use compound pages for f2fs metadata, so this basically just
> > casts the page to a struct folio.
> >
> > folio_mapping() is just like the old page_mapping() (see commit
> > 2f52578f9c64).  Unless you've done something like set the swapcache
> > bit on your metadata page, it's just going to return folio->mapping
> > (ie the same as page->mapping).
>
> Hmm, I've never seen this call stack before, so simply started to suspect
> folio.

I'm afraid this is a f2fs bug... :(

folio wasn't merged at the first report time (5.14-rc2).

https://syzkaller.appspot.com/bug?extid=07ff38c9c93ca170de07

I doubt the direct reason of panic may be the same as the one of bug
reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=215231

However, I still can't figure out why meta_inode's a_ops will change
to f2fs_meta_aops.

Thanks,

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