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

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Date:   Sun, 12 Dec 2021 17:15:00 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     syzbot <syzbot+7cd473c2cac13fd2dd72@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [f2fs-dev] [syzbot] BUG: unable to handle kernel NULL pointer
 dereference in folio_mark_dirty

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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